 |
Marco AFFRONTE
Professore Ordinario
Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica
|
Home |
Curriculum(pdf) |
Didattica |
Pubblicazioni
- Estrazione , accelerazione, condizionamento di correnti di elettroni da plasma generato da catodo cavo di opportuna costruzione e loro impiego in apparecchiature e sistemi industriali di irraggiamento di elettroni
[Brevetto]
Cirri, G. F.; Prudenziati, M.; Affronte, Marco; Campani, M.
abstract
Estrazione , accelerazione, condizionamento di correnti di elettroni da plasma generato da catodo cavo di opportuna costruzione e loro impiego in apparecchiature e sistemi industriali di irraggiamento di elettroni
2024
- Quantum sensing of magnetic fields with molecular spins
[Articolo su rivista]
Bonizzoni, C.; Ghirri, A.; Santanni, F.; Affronte, M.
abstract
Spins are prototypical systems with the potential to probe magnetic fields down to the atomic scale limit. Exploiting their quantum nature through appropriate sensing protocols allows to enlarge their applicability to fields not always accessible by classical sensors. Here we first show that quantum sensing protocols for AC magnetic fields can be implemented with molecular spin ensembles embedded into hybrid quantum circuits. We then show that, using only echo detection at microwave frequency and no optical readout, Dynamical Decoupling protocols synchronized with the AC magnetic fields can enhance sensitivity up to S ≈ 10^(−10) − 10^(−9) T Hz^(−1/2) with a low (4-5) number of applied pulses. These results paves the way for the development of strategies to exploit molecular spins as quantum sensors.
2023
- Calibration-Free and High-Sensitivity Microwave Detectors Based on InAs/InP Nanowire Double Quantum Dots
[Articolo su rivista]
Cornia, S; Demontis, V; Zannier, V; Sorba, L; Ghirri, A; Rossella, F; Affronte, M
abstract
At the cutting-edge of microwave detection technology, novel approaches which exploit the interaction between microwaves and quantum devices are rising. In this study, microwaves are efficiently detected exploiting the unique transport features of InAs/InP nanowire double quantum dot-based devices, suitably configured to allow the precise and calibration-free measurement of the local field. Prototypical nanoscale detectors are operated both at zero and finite source-drain bias, addressing and rationalizing the microwave impact on the charge stability diagram. The detector performance is addressed by measuring its responsivity, quantum efficiency and noise equivalent power that, upon impedance matching optimization, are estimated to reach values up to approximate to 2000 A W-1, 0.04 and root HZ, respectively. The interaction mechanism between the microwave field and the quantum confined energy levels of the double quantum dots is unveiled and it is shown that these semiconductor nanostructures allow the direct assessment of the local intensity of the microwave field without the need for any calibration tool. Thus, the reported nanoscale devices based on III-V nanowire heterostructures represent a novel class of calibration-free and highly sensitive probes of microwave radiation, with nanometer-scale spatial resolution, that may foster the development of novel high-performance microwave circuitries.
2023
- Coherent Quantum Network of Superconducting Qubits as a Highly Sensitive Detector of Microwave Photons for Searching of Galactic Axions
[Relazione in Atti di Convegno]
Gatti, C.; Affronte, M.; Balanov, A.; Bonizzoni, C.; Brida, G.; Chiariello, F.; Chikhi, N.; Coda, G.; D'Elia, A.; Gioacchino, D. Di; Enrico, E.; Eremin, I.; Ernajes, M.; Il'Ichev, E.; Fasolo, L.; Fistul, M.; Ghirri, A.; Greco, A.; Ligi, C.; Maccarone, G.; Meda, A.; Navez, P.; Oelsner, G.; Rajteri, M.; Rettaroli, A.; Ruggiero, B.; Savel'Ev, S.; Silvestrini, P.; Tocci, S.; Ustinov, A.; Vanacore, P.; Zagoskin, A.; Lisitskiy, M.
abstract
We propose a novel approach to detect a low power microwave signal with a frequency of the order of several GHz based on a coherent collective response of quantum states occurring in a superconducting qubits network (SQN). An SQN composes of a large number of superconducting qubits embedded in a low-dissipative superconducting resonator. Our theory predicts that an SQN interacting with the off-resonance microwave radiation, demonstrates the collective alternating current Stark effect that can be measured even in the limit of single photon counting. A design of the layout of three terminals SQN detectors containing 10 flux qubits weakly coupled to a low-dissipative R-resonator and T-transmission line was developed. The samples were fabricated by Al-based technology with Nb resonator. The SQN detector was tested in terms of microwave measurements of scattering parameters and two-tone spectroscopy. A substantial shift of the frequency position of the transmission coefficient drop induced by a second tone pump signal was observed, and this effect clearly manifests a nonlinear multiphoton interaction between the second-tone microwave pump signal and an array of qubits.
2023
- Coupling Sub-nanoliter BDPA Organic Radical Spin Ensembles with YBCO Inverse Anapole Resonators
[Articolo su rivista]
Bonizzoni, Claudio; Maksutoglu, Maksut; Ghirri, Alberto; van Tol, John; Rameev, Bulat; Affronte, Marco
abstract
We report the development and test of planar microwave Inverse Anapole Resonators (IARs) made of superconducting Yttrium Barium Copper Oxide (YBCO) for electron spin resonance spectroscopy on small samples. We first characterize our resonators in zero field and then by carrying out transmission spectroscopy on a diluted α,γ-bisdiphenylene-β-phenylally (BDPA) organic radical spin ensemble in an applied magnetic field. These IARs allow us to carry out electron spin resonance spectroscopy both in continuous-wave and pulsed-wave mode, and to estimate the spin memory time of BDPA. The comparison with the results obtained for the same sample on typical linear coplanar resonators shows an improvement by ≈2 - up to3 – orders of magnitude in spin sensitivity, with effective sensing volumes below 1 nanoliter. The best sensitivity we achieved is S≈10^7 spin/(Hz)^1/2 in the pulsed-wave regime. These results compare well with similar experiments reported in the literature.
2023
- Ultrastrong Magnon-Photon Coupling Achieved by Magnetic Films in Contact with Superconducting Resonators
[Articolo su rivista]
Ghirri, A.; Bonizzoni, C.; Maksutoglu, M.; Mercurio, A.; Di Stefano, O.; Savasta, S.; Affronte, M.
abstract
Coherent coupling between spin-wave excitations (magnons) and microwave photons in a cavity may disclose new paths to unconventional phenomena as well as novel applications. Here, we present a systematic investigation of yttrium iron garnets (YIG) films on top of coplanar waveguide resonators made of superconducting YBa2Cu3O7 (YBCO). We first show that spin-wave excitations with frequency higher than the Kittel mode can be excited by putting in direct contact a 5-pm-thick YIG film with the YBCO coplanar resonator (cavity frequency & omega;c/27r = 8.65 GHz). With this configuration, we obtain very large values of the collective coupling strength & lambda;/27r & AP; 2 GHz and cooperativity C = 5 x 104. Transmission spectra are analyzed by a modified Hopfield model for which we provide an exact solution that allows us to well reproduce spectra by introducing a limited number of free parameters. It turns out that the coupling of the dominant magnon mode with photons exceeds 0.2 times the cavity frequency, thus demonstrating the achievement of the ultrastrong-coupling regime with this architecture. Our analysis also shows a vanishing contribution of the diamagnetic term, which is a peculiarity of pure spin systems.
2022
- Machine-Learning-Assisted Manipulation and Readout of Molecular Spin Qubits
[Articolo su rivista]
Bonizzoni, Claudio; Tincani, Mirco; Santanni, Fabio; Affronte, Marco
abstract
Machine learning finds application in the quantum control and readout of qubits. In this work we apply artificial neural networks to assist the manipulation and the readout of a prototypical molecular spin qubit-an oxovanadium(IV) moiety-in two experiments designed to test the amplitude and the phase recognition, respectively. We first successfully use an artificial network to analyze the output of a storage-retrieval protocol with four input pulses to recognize the echo positions and, with further post selection on the results, to infer the initial input pulse sequence. We then apply an artificial neural network to ascertain the phase of the experimentally measured Hahn echo, showing that it is possible to correctly detect its phase and to recognize additional single-pulse phase shifts added during manipulation.
2022
- Modelling Studies of Magnetostatic Modes in Hybrid MW-YIG Structures
[Relazione in Atti di Convegno]
Maksutoglu, M.; Ghirri, A.; Yorulmaz, S. C.; Yildiz, F.; Affronte, M.; Rameev, B.
abstract
Studies on spin waves (magnons) formed by collective electron spin excitations in magnetic materials have recently gained importance due to their great potential for magnonics and hybrid quantum systems. In this study, the magnetostatic spin-wave modes excited as a result of the coupling between the 2D microwave resonator and the YIG sphere have been investigated computationally. The simulations have been performed using commercial finite element method software (CST Studio Suite). The 2D microwave resonator is realized on the ground plane of the broadband microstrip structure by using a low-loss printed circuit board. The calculations reveal the Q factor to be about 300 by optimizing the resonance frequency to 7 GHz. The most interesting result of simulations is the observation that the magneto-static spin-wave modes of the YIG sphere may be excited depending on the geometry of the hybrid structure. A strong interaction between the YIG and the microwave resonator observed in the dispersion curves of these modes reveals the realization of the strong coupling regime, which is a prerequisite of their use in quantum hybrid devices.
2022
- Ultra-strong coupling in the hybrid quantum system consisting of planar MW resonator and YIG material
[Relazione in Atti di Convegno]
Maksutoglu, M.; Ghirri, A.; Bonizzoni, C.; Cinar, K.; Yorulmaz, S. C.; Yildiz, F.; Affronte, M.; Rameev, B.
abstract
Ultra-strong coupling between inverse anapole resonator (IAR) and YIG crystal was studied. FEM simulations and experimental studies showed both coherently and dissipatively coupled magnon-photon modes, revealing high prospects of this hybrid structure for quantum technology.
2021
- Evaluating the use of graphene electrodes in sub-micrometric, high-frequency n-type organic transistors
[Articolo su rivista]
Chianese, F.; Candini, A.; Lumetti, S.; Mishra, N.; Coletti, C.; Affronte, M.; Cassinese, A.
abstract
2021
- Multiscale Charge Transport in van der Waals Thin Films: Reduced Graphene Oxide as a Case Study
[Articolo su rivista]
Kovtun, A.; Candini, A.; Vianelli, A.; Boschi, A.; Dell'Elce, S.; Gobbi, M.; Kim, K. H.; Lara Avila, S.; Samori, P.; Affronte, M.; Liscio, A.; Palermo, V.
abstract
Large area van der Waals (vdW) thin films are assembled materials consisting of a network of randomly stacked nanosheets. The multiscale structure and the two-dimensional (2D) nature of the building block mean that interfaces naturally play a crucial role in the charge transport of such thin films. While single or few stacked nanosheets (i.e., vdW heterostructures) have been the subject of intensive works, little is known about how charges travel through multilayered, more disordered networks. Here, we report a comprehensive study of a prototypical system given by networks of randomly stacked reduced graphene oxide 2D nanosheets, whose chemical and geometrical properties can be controlled independently, permitting to explore percolated networks ranging from a single nanosheet to some billions with room-temperature resistivity spanning from 10-5 to 10-1 ω·m. We systematically observe a clear transition between two different regimes at a critical temperature T*: Efros-Shklovskii variable-range hopping (ES-VRH) below T∗ and power law behavior above. First, we demonstrate that the two regimes are strongly correlated with each other, both depending on the charge localization length ζ, calculated by the ES-VRH model, which corresponds to the characteristic size of overlapping sp2 domains belonging to different nanosheets. Thus, we propose a microscopic model describing the charge transport as a geometrical phase transition, given by the metal-insulator transition associated with the percolation of quasi-one-dimensional nanofillers with length ζ, showing that the charge transport behavior of the networks is valid for all geometries and defects of the nanosheets, ultimately suggesting a generalized description on vdW and disordered thin films.
2021
- Tetrairon(II) extended metal atom chains as single-molecule magnets
[Articolo su rivista]
Nicolini, A; Affronte, M; SantaLucia, Dj; Borsari, M; Cahier, B; Caleffi, M; Ranieri, A; Berry, Jf; Cornia, A
abstract
Iron-based extended metal atom chains (EMACs) are potentially high-spin molecules with axial magnetic anisotropy and thus candidate single-molecule magnets (SMMs). We herein compare the tetrairon(II), halide-capped complexes [Fe4(tpda)3Cl2] (1Cl) and [Fe4(tpda)3Br2] (1Br), obtained by reacting iron(II) dihalides with [Fe2(Mes)4] and N2,N6-di(pyridin-2-yl)pyridine-2,6-diamine (H2tpda) in toluene, under strictly anhydrous and anaerobic conditions (HMes = mesitylene). Detailed structural, electrochemical and Mossbauer data are presented along with direct-current (DC) and alternating-current (AC) magnetic characterizations. DC measurements revealed similar static magnetic properties for the two derivatives, with chiMT at room temperature above that for independent spin carriers, but much lower at low temperature. The electronic structure of the iron(II) ions in each derivative was explored by ab initio (CASSCF-NEVPT2-SO) calculations, which showed that the main magnetic axis of all metals is directed close to the axis of the chain. The outer metals, Fe1 and Fe4, have an easy-axis magnetic anisotropy (D = -11 to -19 cm-1, |E/D| = 0.05-0.18), while the internal metals, Fe2 and Fe3, possess weaker hard-axis anisotropy (D = 8-10 cm-1, |E/D| = 0.06-0.21). These single-ion parameters were held constant in the fitting of DC magnetic data, which revealed ferromagnetic Fe1-Fe2 and Fe3-Fe4 interactions and antiferromagnetic Fe2-Fe3 coupling. The competition between super-exchange interactions and the large, noncollinear anisotropies at metal sites results in a weakly magnetic non-Kramers doublet ground state. This explains the SMM behavior displayed by both derivatives in the AC susceptibility data, with slow magnetic relaxation in 1Br being observable even in zero static field.
2021
- Transmission Spectroscopy of Molecular Spin Ensembles in the Dispersive Regime
[Articolo su rivista]
Bonizzoni, C.; Ghirri, A.; Nakazawa, S.; Nishida, S.; Sato, K.; Takui, T.; Affronte, M.
abstract
The readout in the dispersive regime is originally developed—and it is now largely exploited—for non-demolitive measurement of super- and semiconducting qubits. More recently it has been successfully applied to probe collective spin excitations in ferro(i)magnetic bulk samples or collections of paramagnetic spin centers embedded into microwave cavities. The use of this readout technique within a semiclassical limit of excitation is only marginally investigated although it holds for a wide class of problems, including advanced magnetic resonance techniques. In this work, the coupling between a coplanar microwave resonator and diphenyl-nitroxide organic radical diluted in a fully deuterated benzophenone single crystal is investigated. Two-tone transmission spectroscopy experiments demonstrate the possibility to reconstruct the spectrum of the spin system with little loss of sensitivity with respect to the resonant regime. Likewise, pulse sequences of detuned microwave frequency allow the measurement of the spin-lattice relaxation time (T1). The independent tunability of the probe and the drive power enables one to adjust the signal-to-noise ratio of the spectroscopy. These results suggest that electron spin dispersive spectroscopy can be used as a complementary tool of electron spin resonance to investigate the spin response.
2020
- Aggregation of a Giant Bean-like Mn26Dy6 Heterometallic Oxo-Hydroxo-Carboxylate Nanosized Cluster from a Hexanuclear Mn6 Precursor
[Articolo su rivista]
Darii, M.; Kravtsov, V. C.; Kramer, K.; Hauser, J.; Decurtins, S.; Liu, S. -X.; Affronte, M.; Baca, S. G.
abstract
Starting from a well-known hexanuclear mixed-valent manganese cluster, we report a synthetic protocol for its extension toward very large 3d-4f clusters. The exemplary case study demonstrates that a combination of such Mn6 clusters with a DyIII salt and an auxiliary triazine-type ligand yields a large bean-like, heterometallic dotriacontanuclear mixed-valent nanocluster [Mn26Dy6O16(OH)12(O2CCHMe2)42]. This compound, which possesses a molecular C2 symmetry and crystallizes in a dense packing arrangement without inclusion of solvents, comprises 16 MnIII, 10 MnII, and 6 DyIII ions, thus leading to a plurality of intracluster magnetic exchange interactions. Interestingly, this results in the highest-nuclearity Mn-Dy cluster that shows a single-molecule magnetic (SMM) behavior.
2020
- Coupling Nanostructured CsNiCr Prussian Blue Analogue to Resonant Microwave Fields
[Articolo su rivista]
Ghirri, Alberto; Herrero, Christian; Mazérat, Sandra; Mallah, Talal; Moze, Oscar; Affronte, Marco
abstract
Collective spin excitations in magnetically ordered materials are exploited for advanced applications in magnonics and spintronics. In these contexts, conditions for minimizing dissipative effects are sought in order to obtain long living excitations that can be coherently manipulated. Organic and coordination magnetic materials may offer alternative options for their flexibility and low spin-orbit effects. Likewise, ferromagnetic nanostructures provide a versatile platform for hybrid architectures, yet downsizing affects the dynamics of magnetic excitations and needs to be controlled. Here we report a systematic investigation on insulating CsNiCr Prussian blue analogue with different degree of nanostructuring. Combining complementary microwave spectroscopic techniques, we performed magnetic resonance in a wide temperature range across the bulk ferromagnetic transition occurring at TC=90 K. This allows us to monitor key parameters of the spin dynamics through different types of nanostructured samples. We found that, below TC, the Gilbert damping parameter of 10 nm nanoparticles compares well (10-3) with values reported for prototypical inorganic analogues (YIG). Strong coupling with the microwave field of a planar microstrip resonator is then observed for bulk CsNiCr as well as for mutually interacting NPs. These results clarify conditions for the coherent manipulation of collective spin degrees of freedom in nanostructured coordination materials.
2020
- Hydrodynamical description for magneto-transport in the strange metal phase of Bi-2201
[Articolo su rivista]
Amoretti, A.; Meinero, M.; Brattan, D. K.; Caglieris, F.; Giannini, E.; Affronte, M.; Hess, C.; Buechner, B.; Magnoli, N.; Putti, M.
abstract
High-temperature superconductors are strongly coupled systems which present a complicated phase diagram with many coexisting phases. This makes it difficult to understand the mechanism which generates their singular transport properties. Hydrodynamics, which mostly relies on the symmetries of the system without referring to any specific microscopic mechanism, constitutes a promising framework to analyze these materials. In this paper we show that, in the strange metal phase of the cuprates, a whole set of transport coefficients are described by a universal hydrodynamic framework once one accounts for the effects of quantum critical charge-density waves. We corroborate our theoretical prediction by measuring the DC transport properties of Bi-2201 close to optimal doping, proving the validity of our approach. Our argument can be used as a consistency check to understand the universality class governing the behavior of high-temperature cuprate superconductors.
2020
- Microwave photon detectors based on semiconducting double quantum dots
[Articolo su rivista]
Ghirri, A.; Cornia, S.; Affronte, M.
abstract
Detectors of microwave photons find applications in different fields ranging from security to cosmology. Due to the intrinsic difficulties related to the detection of vanishingly small energy quanta ¯hω, significant portions of the microwave electromagnetic spectrum are still uncovered by suitable techniques. No prevailing technology has clearly emerged yet, although different solutions have been tested in different contexts. Here, we focus on semiconductor quantum dots, which feature wide tunability by external gate voltages and scalability for large architectures. We discuss possible pathways for the development of microwave photon detectors based on photon-assisted tunneling in semiconducting double quantum dot circuits. In particular, we consider implementations based on either broadband transmission lines or resonant cavities, and we discuss how developments in charge sensing techniques and hybrid architectures may be beneficial for the development of efficient photon detectors in the microwave range.
2020
- S-functionalized tripods with monomethylene spacers: Routes to tetrairon(III) single-molecule magnets with ultrashort tethering groups
[Articolo su rivista]
Cornia, A.; Danieli, C.; Meglioli, F.; Tancini, E.; Nicolini, A.; Rodriguez-Douton, M. J.; Barra, A. -L.; Affronte, M.; Sessoli, R.
abstract
The organization of single-molecule magnets (SMMs) on surfaces is a mainstream research path in molecular magnetism. Of special importance is the control of grafting geometry in chemisorbed monolayers on metal surfaces. We herein present the synthesis, solid-state structure, and magnetic characterization of propeller-like tetrairon(III) SMMs containing the shortest-reported tethering groups for gold surfaces. Functionalization of molecular structure is attained using 2-R-2-(hydroxymethyl)propane-1,3-diol tripodal proligands (H3LR). The R substituents comprise a monomethylene spacer and three different terminations known to act as stable precursors of S-Au bonds (R = CH2SCN, CH2SAc and CH2SSnBu). These chemical groups are shown to be chemically compatible with the tetrairon(III) core and to afford fully-functional SMMs in crystalline form and in fair to excellent yields.
2020
- Storage and retrieval of microwave pulses with molecular spin ensembles
[Articolo su rivista]
Bonizzoni, C.; Ghirri, A.; Santanni, F.; Atzori, M.; Sorace, L.; Sessoli, R.; Affronte, M.
abstract
Hybrid architectures combining complementary quantum systems will be largely used in quantum technologies and the integration of different components is one of the key issues. Thanks to their long coherence times and the easy manipulation with microwave pulses, electron spins hold a potential for the realization of quantum memories. Here, we test diluted oxovanadium tetraphenyl porphyrin (VO(TPP)) as a prototypical molecular spin system for the Storage/Retrieval of microwave pulses when embedded into planar superconducting microwave resonators. We first investigate the efficiency of several pulse sequences in addressing the spins. The Carr-Purcell and the Uhrig Dynamical Decoupling enhance the memory time up to three times with three π pulses. We then successfully store and retrieve trains of up to 5 small pulses by using a single recovery pulse. These results demonstrate the memory capabilities of molecular spin ensembles when embedded into quantum circuits.
2020
- Suppression of the morphology mismatch at graphene/n-type organic semiconductor interfaces: a scanning Kelvin probe force microscopy investigation
[Articolo su rivista]
Chianese, F.; Chiarella, F.; Barra, M.; Candini, A.; Affronte, M.; Cassinese, A.
abstract
Contact resistance effects in n-type organic field-effect transistors (OFETs) based on perylene-diimide thin films and monolayer CVD graphene electrodes have been investigated by using Scanning Kelvin Probe Force Microscopy (SKPFM). SKPFM voltage profiles were acquired as a function of the applied drain-source (Vds) and gate-source (Vgs) voltages and in the temperature range between 300 and 360 K. The results were compared with those obtained for hybrid devices bearing contemporarily a gold and a graphene electrode, in order to highlight the specific behaviors of the graphene/organic and gold/organic interfaces. Despite the fact that overall electrical performances of devices based on gold and graphene contacts are fully comparable, the SKPFM analysis demonstrates unambiguously that the physical mechanisms driving the charge injection and extraction phenomena are distinctive based on the electrode type. While for OFETs with gold electrodes, the RC effect is mainly ascribable to the degraded quality of the charge transport in the semiconducting film regions close to the electrodes, for graphene-based devices, it is related to the presence of a Schottky-like barrier at the injecting electrode. This journal is
2020
- Uncollapsed LaFe2 As2 phase: Compensated, highly doped, electron-phonon-coupled, iron-based superconductor
[Articolo su rivista]
Pallecchi, I.; Iyo, A.; Ogino, H.; Affronte, M.; Putti, M.
abstract
The recently discovered LaFe2As2 superconducting compound, member of the 122 family of iron pnictide superconductors, becomes superconducting below TcGëê13K, yet its nominal doping apparently places it in the extreme overdoped limit, where superconductivity should be suppressed. In this work, we investigate the normal state of magneto- and thermoelectric transport and specific heat of this compound. The experimental data are consistent with the presence of highly compensated electron and hole bands, with Gê+0.42 electrons per unit cell just above Tc, and high effective masses Gê+3m0. The temperature dependence of transport properties strongly resembles that of conventional superconductors, pointing to a key role of electron-phonon coupling. From this evidence, LaFe2As2 can be regarded as the connecting compound between unconventional and conventional superconductors.
2019
- Aggregation of [Ln III 12] clusters by the dianion of 3-formylsalicylic acid. Synthesis, crystal structures, magnetic and luminescence properties
[Articolo su rivista]
Dinca, A. S.; Mindru, A.; Dragancea, D.; Tiseanu, C.; Shova, S.; Cornia, Samuele; Carrella, L. M.; Rentschler, E.; Affronte, M.; Andruh, M.
abstract
Three isostrucutral dodecanuclear clusters with the general formula [Ln(12)(fsa)(12)(mu f(3)-OH)(12)(DMF)(12)]center dot nDMF (fsa(2-) is the dianion of 3-formylsalicylic acid; Ln = Eu 1, Gd 2, Dy 3) have been obtained from the reaction of fromylsalicyclic acid (H(2)fsa), tetrabutylammonium hydroxide and Ln(NO3)(3)center dot 6H(2)O in methanol/DMF. Their structure consists of four vertex-sharing heterocubanes. Each heterocubane unit is assembled from four Ln(III) ions, three mu(3)-OH groups and one mu(3)-oxygen atom arising from the fsa(2-) carboxylato group. The photophysical properties of the europium derivative investigated at both 300 and 80 K sustain a relative intense emission obtained under low power LED excitation at 420 nm. The dysprosium derivative shows a SMM behavior with an effective energy barrier U-eff of 22.9 cm(-1), while the thermodynamical properties of Gd-12 confirm a large magnetocaloric effect: S(7 T) -S(0 T) = 20R = 166 J mol(-1) K), typical for isotropic Gd-III derivatives, with Delta S = S(7 T) - S(0 T) = 1.7R for each Gd-III ion.
2019
- Color Sensitive Response of Graphene/Graphene Quantum Dot Phototransistors
[Articolo su rivista]
Fantuzzi, P.; Candini, A.; Chen, Q.; Yao, X.; Dumslaff, T.; Mishra, N.; Coletti, C.; Mullen, K.; Narita, A.; Affronte, M.
abstract
We present the fabrication and characterization of all-carbon phototransistors made of graphene three terminal devices, coated with atomically precise graphene quantum dots (GQD). Chemically synthesized GQDs are the light absorbing materials, while the underlying chemical vapor deposition (CVD)-grown graphene layer acts as the charge transporting channel. We investigated three types of GQDs with different sizes and edge structures, having distinct and characteristic optical absorption in the UV-vis range. The photoresponsivity exceeds 106 A/W for vanishingly small incident power (<10-12 W), comparing well with state of the art sensitized graphene photodetectors. More importantly, the photoresponse is determined by the specific absorption spectrum of each GQD, exhibiting the maximal responsivity at the wavelengths corresponding to the absorption maxima. Overall this behavior can be ascribed to the efficient and selective absorption of light by the GQDs, followed by a charge transfer to graphene, a mechanism known as the photogating effect. Our results suggest the use of graphene/GQD devices as valuable photodetectors for application where color sensitivity is required.
2019
- CoTPP molecules deposited on graphene/Ni (111): Quenching of the antiferromagnetic interaction induced by gold intercalation
[Articolo su rivista]
Corradini, V.; Candini, A.; Klar, D.; Biagi, R.; De Renzi, V.; Lodi Rizzini, A.; Cavani, N.; Del Pennino, U.; Wende, H.; Otero, E.; Affronte, M.
abstract
In this work, we investigated the effect of Au-intercalation on the magnetic coupling between a sub-monolayer of Co-Tetraphenylporphyrin molecules and a graphene-covered Ni(111) single crystal. Using x-ray absorption spectroscopy and x-ray magnetic circular dichroism, the element-specific magnetization and its field dependence were probed. Cobalt strongly couples antiferromagnetically to the nickel substrate, also through the graphene layer. The intercalation of graphene with gold leads to a complete removal of this coupling. Published under license by AIP Publishing.
2019
- Microwave-assisted reversal of a single electron spin
[Articolo su rivista]
Godfrin, C.; Lumetti, Stefano; Biard, H.; Bonet, E.; Klyatskaya, S.; Ruben, M.; Candini, A.; Affronte, M.; Wernsdorfer, W.; Balestro, F.
abstract
The dynamics of a quantum system in a dissipative environment presents a rich and still largely unexplored phenomenology that is relevant for the control of quantum devices. The simplest problem can be modeled as a two-level system-such as a spin-in contact with a thermal bath. Here, we present experiments in which we monitor the spin reversal in a TbPc2 single-molecule spin transistor, and we show that the application of microwave pulses can cause the spin to flip between its two lowest-lying states (vertical bar up arrow > and vertical bar down arrow >) at a rate that increases with increasing duration and power of the pulses. This process is neither resonant nor coherent with the microwave pulses. Moreover, an asymmetry appears in the vertical bar down arrow >->vertical bar up arrow > and vertical bar up arrow >->vertical bar down arrow > transition probabilities, suggesting that the process occurs out of equilibrium. We explain the experimental results and provide an estimate of the local temperature increase induced by the microwave pulses by means of a model that takes into account the energy exchange between the single TB3+ electron spin and the local environment. Published under license by AIP Publishing.
2019
- Microwave-Assisted Tunneling in Hard-Wall InAs/InP Nanowire Quantum Dots
[Articolo su rivista]
Cornia, S.; Rossella, F.; Demontis, V.; Zannier, V.; Beltram, F.; Sorba, L.; Affronte, M.; Ghirri, A.
abstract
With downscaling of electronic circuits, components based on semiconductor quantum dots are assuming increasing relevance for future technologies. Their response under external stimuli intrinsically depend on their quantum properties. Here we investigate single-electron tunneling in hard-wall InAs/InP nanowires in the presence of an off-resonant microwave drive. Our heterostructured nanowires include InAs quantum dots (QDs) and exhibit different tunnel-current regimes. In particular, for source-drain bias up to few mV Coulomb diamonds spread with increasing contrast as a function of microwave power and present multiple current polarity reversals. This behavior can be modelled in terms of voltage fluctuations induced by the microwave field and presents features that depend on the interplay of the discrete energy levels that contribute to the tunneling process.
2019
- Towards quantum sensing with molecular spins
[Articolo su rivista]
Troiani, F.; Ghirri, A.; Paris, M. G. A.; Bonizzoni, C.; Affronte, M.
abstract
Molecular spins have shown genuine quantum properties, both as collections of independent units and as individual objects. Their properties and performances can be engineered at molecular level, while advanced technologies to coherently manipulate magnetic objects and to address them with unprecedented spatial and energy resolution have emerged in the last years. Here we address the question on how to exploit quantum features of molecular spins for quantum sensing. To this end, we summarise some basic ideas and discuss some examples where molecular spins can play a role in the field.
2018
- Coherent coupling of molecular spins with microwave photons in planar superconducting resonators
[Articolo su rivista]
Bonizzoni, Claudio; Ghirri, Alberto; Affronte, Marco
abstract
Within the quest for solid state quantum systems to be
used for fundamental as well as applied research, molecular
spins have recently emerged as a versatile platform with
interesting performances in terms of quantum coherence and
correlation. Molecular units provide well defined environment
to electronic spins and they represent elementary bricks for
complex nano-architectures and nano-devices. Here we review
our recent efforts and results on their efficient integration in
circuit Quantum ElectroDynamics and, more specifically, in
reaching their coherent coupling with microwave photons in
planar resonators. To monitor molecular spin performances
over a wide temperature and magnetic field range we have
first developed microwave planar resonators made of high
Tc superconductors, obtaining excellent performances up
to liquid Nitrogen temperature and in magnetic fields up to
7 Tesla. Ensembles of different molecular spins systems are
then systematically tested. The regime of high spin-photon
cooperativity is achieved with molecular spins diluted in nonmagnetic
matrix at 0.5 K, while the strong coupling regime is
observed with concentrated samples of organic radicals up to
50 K. The possibility to create coherent states among distinct
spin ensembles is further explored in similar spectroscopic
experiments. These results show that molecular spins can be
efficiently integrated in quantum devices.
2018
- High spin cycles: topping the spin record for a single molecule verging on quantum criticality
[Articolo su rivista]
Baniodeh, Amer; Magnani, Nicola; Lan, Yanhua; Buth, Gernot; Anson, Christopher E.; Richter, Johannes; Affronte, Marco; Schnack, Jürgen; Powell, Annie K.
abstract
The cyclisation of a short chain into a ring provides fascinating scenarios in terms of transforming a finite array of spins into a quasiinfinite
structure. If frustration is present, theory predicts interesting quantum critical points, where the ground state and thus lowtemperature
properties of a material change drastically upon even a small variation of appropriate external parameters. This can be
visualised as achieving a very high and pointed summit where the way down has an infinity of possibilities, which by any parameter
change will be rapidly chosen, in order to reach the final ground state. Here we report a mixed 3d/4f cyclic coordination cluster that
turns out to be very near or even at such a quantum critical point. It has a ground state spin of S = 60, the largest ever observed for
a molecule (120 times that of a single electron). [Fe10Gd10(Me-tea)10(Me-teaH)10(NO3)10]·20MeCN forms a nano-torus with
alternating gadolinium and iron ions with a nearest neighbour Fe–Gd coupling and a frustrating next-nearest neighbour Fe–Fe
coupling. Such a spin arrangement corresponds to a cyclic delta or saw-tooth chain, which can exhibit unusual frustration effects. In
the present case, the quantum critical point bears a ‘flatland’ of tens of thousands of energetically degenerate states between
which transitions are possible at no energy costs with profound caloric consequences. Entropy-wise the energy flatland translates
into the pointed summit overlooking the entropy landscape. Going downhill several target states can be reached depending on the
applied physical procedure which offers new prospects for addressability.
2018
- Linear conduction in N-type organic field effect transistors with nanometric channel lengths and graphene as electrodes
[Articolo su rivista]
Chianese, F.; Candini, A.; Affronte, M.; Mishra, N.; Coletti, C.; Cassinese, A.
abstract
In this work, we test graphene electrodes in nanometric channel n-type Organic Field Effect Transistors (OFETs) based on thermally evaporated thin films of the perylene-3,4,9,10-tetracarboxylic acid diimide derivative. By a thorough comparison with short channel transistors made with reference gold electrodes, we found that the output characteristics of the graphene-based devices respond linearly to the applied bias, in contrast with the supralinear trend of gold-based transistors. Moreover, short channel effects are considerably suppressed in graphene electrode devices. More specifically, current on/off ratios independent of the channel length (L) and enhanced response for high longitudinal biases are demonstrated for L down to ∼140 nm. These results are rationalized taking into account the morphological and electronic characteristics of graphene, showing that the use of graphene electrodes may help to overcome the problem of Space Charge Limited Current in short channel OFETs.
2018
- Microwave dual-mode resonators for coherent spin-photon coupling
[Articolo su rivista]
Bonizzoni, C.; Troiani, F.; Ghirri, A.; Affronte, M.
abstract
We implement superconducting Yttrium barium copper oxide planar resonators with two fundamental modes for circuit quantum electrodynamics experiments. We first demonstrate good tunability in the resonant microwave frequencies and in their interplay, as emerges from the dependence of the transmission spectra on the device geometry. We then investigate the magnetic coupling of the resonant modes with bulk samples of 2,2-diphenyl-1-picrylhydrazyl organic radical spins. The transmission spectroscopy performed at low temperature shows that the coherent spin-photon coupling regime with the spin ensembles can be achieved by each of the resonator modes. The analysis of the results within the framework of the input-output formalism and by means of entropic measures demonstrates coherent mixing of the degrees of freedom corresponding to two remote spin ensembles and, with a suitable choice of the geometry, the approaching of a regime with spin-induced mixing of the two photon modes.
2018
- Probing magnetic coupling between LnPc2 (Ln=Tb, Er) molecules and graphene / Ni(111) substrate with and without Au-intercalation: role of the dipolar field
[Articolo su rivista]
Corradini, Valdis; Candini, Andrea; Klar, David; Biagi, Roberto; de Renzi, Valentina; Lodi Rizzini, Alberto; Cavani, Nicola; del Pennino, Umberto; Klyatskaya, Svetlana; Ruben, Mario; Velez-Fort, Emilio; Kummer, Kurt; Brookes, Nicholas B.; Gargiani, Pierluigi; Wende, Heiko; Affronte, Marco
abstract
Lanthanides (Ln) bis-phthalocyanine (Pc), the so called LnPc2 double decker, are promising
class of molecules, with well-defined magnetic anisotropy. In this work, we investigate the
magnetic properties of LnPc2 molecules UHV-deposited on the graphene/Ni(111) substrate, and
how they modify when an Au layer is intercalated between the Ni and graphene. X-ray
absorption spectroscopy (XAS), linear and magnetic circular dichroism (XLD and XMCD)
were used to characterize the systems and probe the magnetic coupling between LnPc2
molecules and the Ni substrate through graphene, both gold intercalated or not. Two types of
LnPc2 molecules (Ln=Tb, Er) with a different magnetic anisotropy (easy-axis for Tb, easyplane for Er) were considered. XMCD shows an antiferromagnetic coupling between Ln and Ni(111) even in the presence of the graphene interlayer. Au intercalation causes the vanishing
of the interaction between Tb and Ni(111). On the contrary, in the case of ErPc2, we found that
the gold intercalation does not perturb the magnetic coupling. These results, combined with the
magnetic anisotropy of the systems, suggests the possible importance of the magnetic dipolar
field contribution in determining the magnetic behaviour.
2018
- Radical-lanthanide ferromagnetic interaction in a TbIII bis-phthalocyaninato complex
[Articolo su rivista]
Komijani, Dorsa; Ghirri, Alberto; Bonizzoni, Claudio; Klyatskaya, Svetlana; Moreno-Pineda, Eufemio; Ruben, Mario; Soncini, Alessandro; Affronte, Marco; Hill, Stephen
abstract
Recent studies have highlighted the importance of organic ligands in the field of molecular spintronics, via
which delocalized electron-spin density can mediate magnetic coupling to otherwise localized 4f moments
of lanthanide ions, which show tremendous potential for single-molecule device applications. To this end, highfield/
high-frequency electron paramagnetic resonance (EPR) spectroscopy is employed to study a neutral terbium
bis-phthalocyaninato metalorganic complex, [TbPc2]0, with the aim of understanding the magnetic interaction
between the Ising-like moment of the lanthanide ion and the unpaired spin density on the coordinating organic
radical ligand. Themeasurements were performed on a previously unknown[TbPc2]0 structural phase crystallizing
in the Pnma space group. EPR measurements on powder samples of [TbPc2]0 reveal an anisotropic spectrum,
which is attributed to the spin- 1
2 radical coupled weakly to the EPR-silent TbIII ion. Extensive double-axis rotation
studies on a single crystal reveal two independent spin- 1
2 signals with differently oriented (albeit identical) uniaxial
g-tensors, in complete agreement with x-ray structural studies that indicate two molecular orientations within
the unit cell. The easy-axis nature of the radical EPR spectra thus reflects the coupling to the Ising-like TbIII
moment. This is corroborated by studies of the isostructural [YPc2]0 analog (where Y is nonmagnetic yttrium),
which gives a completely isotropic radical EPR signal. The experimental results for the terbium complex are well
explained on the basis of an effective model that introduces a weak ferromagnetic Heisenberg coupling between
an isotropic spin- 1/2 and an anisotropic spin-orbital moment, J = 6, that mimics the known, strong easy-axis
Tb · · · Pc2 crystal-field interaction.
2017
- Addressing single molecular spin with graphene based nano-architectures.
[Capitolo/Saggio]
Candini, A.; Lumetti, S.; Balestro, F.; Wernsdorfer, W.; Affronte, M.
abstract
Finding reliable methods to exploit molecular degrees of freedom represents an intriguing problem involving the control of new mechanisms at the nano-scale and several technological challenges. Here we report a novel approach to address single molecular spin embedded in an electronic circuit. Our devices make use of molecules with well-defined magnetic anisotropy (TbPc2) embedded in nano-gapped electrodes obtained by electro-burning graphene layers. Such devices work as molecular spin transistors allowing the detection of the Tb spin flip during the sweep of an external magnetic field. The spin read out is made by the molecular quantum dot that, in turns, is driven by an auxiliary gate voltage. In the general context of (spin-)electronics, these results demonstrate that: 1) molecular quantum dots can be used as ultra-sensitive detectors for spin flip detection and 2) the use of graphene electrodes as platform to contact organo-metallic molecule is a viable route to design more complex nano-architectures.
2017
- Coherent coupling between Vanadyl Phthalocyanine spin ensemble and microwave photons: Towards integration of molecular spin qubits into quantum circuits
[Articolo su rivista]
Bonizzoni, C.; Ghirri, A.; Atzori, M.; Sorace, L.; Sessoli, Roberta; Affronte, M.
abstract
Electron spins are ideal two-level systems that may couple with microwave photons so that, under specific conditions, coherent spin-photon states can be realized. This represents a fundamental step for the transfer and the manipulation of quantum information. Along with spin impurities in solids, molecular spins in concentrated phases have recently shown coherent dynamics under microwave stimuli. Here we show that it is possible to obtain high cooperativity regime between a molecular Vanadyl Phthalocyanine (VOPc) spin ensemble and a high quality factor superconducting YBa2Cu3O7 (YBCO) coplanar resonator at 0.5 K. This demonstrates that molecular spin centers can be successfully integrated in hybrid quantum devices.
2017
- High Photoresponsivity in Graphene Nanoribbon Field Effect Transistor Devices Contacted With Graphene Electrodes
[Articolo su rivista]
Candini, Andrea; Martini, Leonardo; Chen, Zongping; Mishra, Neeraj; Convertino, Domenica; Coletti, Camilla; Narita, Akimitsu; Feng, Xinliang; Müllen, Klaus; Affronte, Marco
abstract
Ultra-narrow graphene nanoribbons (GNRs) with atomically precise structures are considered a promising class of materials for the realization of optoelectronic and photonic devices with improved functionalities. Here we report the opto-electronic characterization of a field effect transistor devices made of a layer of bottom-up synthesized GNRs contacted with multilayer graphene electrodes, showing high photoresponsivity of 5 × 105 A/W for small incident power in the visible-UV range. Our results show that combining the properties of intrinsic graphene with that of semiconducting GNRs is a viable route to realize novel devices for optoelectronic and sensing applications.
2017
- Landau-Zener Transition in a Continuously Measured Single-Molecule Spin Transistor
[Articolo su rivista]
Troiani, F.; Godfrin, C.; Thiele, S.; Balestro, F.; Wernsdorfer, W.; Klyatskaya, S.; Ruben, M.; Affronte, M.
abstract
We monitor the Landau-Zener dynamics of a single-ion magnet inserted into a spin-transistor geometry. For increasing field-sweep rates, the spin reversal probability shows increasing deviations from that of a closed system. In the low-conductance limit, such deviations are shown to result from a dephasing process. In particular, the observed behaviors are successfully simulated by means of an adiabatic master equation, with time averaged dephasing (Lindblad) operators. The time average is tentatively interpreted in terms of the finite time resolution of the continuous measurement.
2017
- Molecular Spins in the Context of Quantum Technologies
[Articolo su rivista]
Ghirri, Alberto; Candini, Andrea; Affronte, Marco
abstract
Molecular spins have shown interesting quantum features which make them potential candidates for the implementation of quantum information processing. New challenges related to possible applications in broader class of quantum technologies are currently under discussion. Here, we revisit some key features trying to learn something from experiences in near fields.
2017
- Thin Layers of Molecular Magnets
[Capitolo/Saggio]
Cornia, A.; Talham, D. R.; Affronte, M.
abstract
Thin films of magnetic molecular materials are increasingly under investigation for such diverse applications as molecular spintronics, quantum computation and on-chip magnetic refrigeration. We herein review recent progress in the preparation and characterization of films of magnetic molecules or network solids with thickness ranging from several hundred nanometers down to monolayers or submonolayers. Particular emphasis is placed on the design, processing and magnetic study of the films, and on changes in magnetic response resulting from reduced dimensionality.
2016
- Coherently coupling distinct spin ensembles through a high-Tc superconducting resonator
[Articolo su rivista]
Ghirri, Alberto; Bonizzoni, Claudio; Troiani, Filippo; Buccheri, N.; Beverina, L.; Cassinese, A.; Affronte, Marco
abstract
The problem of coupling multiple spin ensembles through cavity photons is revisited by using (3,5-dichloro-4- pyridyl)bis(2,4,6-trichlorophenyl) methyl (PyBTM) organic radicals and a high-T-c superconducting coplanar resonator. An exceptionally strong coupling is obtained and up to three spin ensembles are simultaneously coupled. The ensembles are made physically distinguishable by chemically varying the g factor and by exploiting the inhomogeneities of the applied magnetic field. The coherent mixing of the spin and field modes is demonstrated by the observed multiple anticrossing, along with the simulations performed within the input-output formalism, and quantified by suitable entropic measures.
2016
- Coupling molecular spin centers to microwave planar resonators: towards integration of molecular qubits in quantum circuits
[Articolo su rivista]
Bonizzoni, Claudio; Ghirri, Alberto; Bader, K.; Van Slageren, J.; Perfetti, M.; Sorace, L.; Lan, Y.; Fuhr, O.; Ruben, M.; Affronte, Marco
abstract
We present spectroscopic measurements looking for the coherent coupling between molecular magnetic centers and microwave photons. The aim is to find the optimal conditions and the best molecular features to achieve the quantum strong coupling regime, for which coherent dynamics of hybrid photon-spin states take place. To this end, we used a high critical temperature YBCO superconducting planar resonator working at 7.7 GHz and at low temperatures to investigate three molecular mononuclear coordination compounds, namely (PPh4)2[Cu(mnt)2] (where mnt2- = maleonitriledithiolate), [ErPc2]-TBA+ (where pc2- is the phtalocyaninato and TBA+ is the tetra-n-butylammonium cation) and Dy(trensal) (where H3trensal = 2,2′,2′′-tris(salicylideneimino)triethylamine). Although the strong coupling regime was not achieved in these preliminary experiments, the results provided several hints on how to design molecular magnetic centers to be integrated into hybrid quantum circuits.
2016
- Fabrication of three terminal devices by ElectroSpray deposition of graphene nanoribbons
[Articolo su rivista]
Fantuzzi, Paolo; Martini, Leonardo; Candini, A.; Corradini, V.; Del Pennino, Umberto; Hu, Y.; Feng, X.; Müllen, K.; Narita, A.; Affronte, Marco
abstract
Electrospray deposition (ESD) in ambient conditions has been used to deposit graphene nanoribbons (GNRs) dispersed in liquid phase on different types of substrates, including ones suitable for electrical transport. The deposition process was controlled and optimized by using Raman spectroscopy, Scanning Probe Microscopies and Scanning Electron Microscopy. When deposited on graphitic electrodes, GNRs were used as semi-conducting channel in three terminal devices showing gate tunability of the electrical current. These results suggest that ESD technique can be used as an effective tool to deposit chemically synthesized GNRs onto substrates of interest for technological applications.
2016
- Heterodimers of heterometallic rings
[Articolo su rivista]
Timco, Grigore; Marocchi, Simone; Garlatti, Elena; Barker, Claire; Albring, Morten; Bellini, Valerio; Manghi, Franca; Mcinnes, Eric J. L.; Pritchard, Robin G.; Tuna, Floriana; Wernsdorfer, Wolfgang; Lorusso, Giulia; Amoretti, Giuseppe; Carretta, Stefano; Affronte, Marco; Winpenny, Richard E. P.
abstract
Nine new complexes are reported involving linked heterometallic rings; one ring is designed as a ligand for the second, and the compounds produced can be regarded as molecular prototypes for implementing quantum gates featuring two subtly different qubits.
2016
- Relay-Like Exchange Mechanism through a Spin Radical between TbPc2 Molecules and Graphene/Ni(111) Substrates
[Articolo su rivista]
Marocchi, Simone; Candini, Andrea; Klar, David; Van Den Heuvel, Willem; Huang, Haibei; Troiani, Filippo; Corradini, Valdis; Biagi, Roberto; DE RENZI, Valentina; Klyatskaya, Svetlana; Kummer, Kurt; Brookes, Nicholas B.; Ruben, Mario; Wende, Heiko; DEL PENNINO, Umberto; Soncini, Alessandro; Affronte, Marco; Bellini, Valerio
abstract
We investigate the electronic and magnetic properties of TbPc2 single ion magnets adsorbed on a graphene/Ni(111) substrate, by density functional theory (DFT), ab initio complete active space self-consistent field calculations, and X-ray magnetic circular dichroism (XMCD) experiments. Despite the presence of the graphene decoupling layer, a sizable antiferromagnetic coupling between Tb and Ni is observed in the XMCD experiments. The molecule-surface interaction is rationalized by the DFT analysis and is found to follow a relay-like communication pathway, where the radical spin on the organic Pc ligands mediates the interaction between Tb ion and Ni substrate spins. A model Hamiltonian which explicitly takes into account the presence of the spin radical is then developed, and the different magnetic interactions at play are assessed by first-principle calculations and by comparing the calculated magnetization curves with XMCD data. The relay-like mechanism is at the heart of the process through which the spin information contained in the Tb ion is sensed and exploited in carbon-based molecular spintronics devices.
2016
- Single-molecule devices with graphene electrodes
[Articolo su rivista]
Lumetti, Stefano; Candini, Andrea; Godfrin, C.; Balestro, F.; Wernsdorfer, W.; Klyatskaya, S.; Ruben, M.; Affronte, Marco
abstract
Several technological issues have to be faced to realize devices working at the single molecule level. One of the main challenges consists of defining methods to fabricate electrodes to make contact with single molecules. Here, we report the realization of novel spintronic devices made of a TbPc2 single molecule embedded between two nanometer-separated graphene electrodes, obtained by feedback-controlled electroburning. We demonstrate that this approach allows the realisation of devices working at low temperature. With these, we were able to characterize the magnetic exchange coupling between the electronic spin of the Tb3+ magnetic core and the current passing through the molecular system in the Coulomb blockade regime, thus showing that the use of graphene is a promising way forward in addressing single molecules.
2016
- Spin-communication channels between Ln(III) bis-phthalocyanines molecular nanomagnets and a magnetic substrate
[Articolo su rivista]
Candini, A; Klar, D.; Marocchi, S.; Corradini, V.; Biagi, Roberto; DE RENZI, Valentina; DEL PENNINO, Umberto; Troiani, F.; Bellini, V.; Klyatskaya, S.; Ruben, M.; Kummer, K.; Brookes, N. B.; Huang, H.; Soncini, A.; Wende, H.; Affronte, Marco
abstract
Learning the art of exploiting the interplay between different units at the atomic scale is a fundamental step in the realization of functional nano-architectures and interfaces. In this context, understanding and controlling the magnetic coupling between molecular centers and their environment is still a challenging task. Here we present a combined experimental-theoretical work on the prototypical case of the bis(phthalocyaninato)-lanthanide(III) (LnPc 2) molecular nanomagnets magnetically coupled to a Ni substrate. By means of X-ray magnetic circular dichroism we show how the coupling strength can be tuned by changing the Ln ion. The microscopic parameters of the system are determined by ab-initio calculations and then used in a spin Hamiltonian approach to interpret the experimental data. By this combined approach we identify the features of the spin communication channel: the spin path is first realized by the mediation of the external (5d) electrons of the Ln ion, keeping the characteristic features of the inner 4 f orbitals unaffected, then through the organic ligand, acting as a bridge to the external world.
2016
- Synthesis of Graphene Nanoribbons by Ambient-Pressure Chemical Vapor Deposition and Device Integration
[Articolo su rivista]
Chen, Zongping; Zhang, Wen; Palma, Carlos Andres; LODI RIZZINI, Alberto; Liu, Bilu; Abbas, Ahmad; Richter, Nils; Martini, Leonardo; Wang, Xiao Ye; Cavani, Nicola; Lu, Hao; Mishra, Neeraj; Coletti, Camilla; Berger, Reinhard; Klappenberger, Florian; Kläui, Mathias; Candini, Andrea; Affronte, Marco; Zhou, Chongwu; DE RENZI, Valentina; DEL PENNINO, Umberto; Barth, Johannes V.; Räder, Hans Joachim; Narita, Akimitsu; Feng, Xinliang; Müllen, Klaus
abstract
Graphene nanoribbons (GNRs), quasi-one-dimensional graphene strips, have shown great potential for nanoscale electronics, optoelectronics, and photonics. Atomically precise 47 8 GNRs can be "bottom-up" synthesized by surface-assisted assembly of molecular building blocks under ultra-high-vacuum conditions. However, large-scale and efficient synthesis of such GNRs at low cost remains a significant challenge. Here we report an efficient "bottom-up" chemical vapor deposition (CVD) process for inexpensive and high-throughput growth of structurally defined GNRs with varying structures under ambient-pressure conditions. The high quality of our CVD-grown GNRs is validated by a combination of different spectroscopic and microscopic characterizations. Facile, large-area transfer of GNRs onto insulating substrates and subsequent device fabrication demonstrate their promising potential as semiconducting materials, exhibiting high current on/off ratios up to 6000 in field-effect transistor devices. This value is 3 orders of magnitude higher than values reported so far for other thin-film transistors of structurally defined GNRs. Notably, on-surface mass spectrometry analyses of polymer precursors provide unprecedented evidence for the chemical structures of the resulting GNRs, especially the heteroatom doping and heterojunctions. These results pave the way toward the scalable and controllable growth of GNRs for future applications.
2015
- Coherent Spin Dynamics in Molecular Cr8Zn Wheels
[Articolo su rivista]
Ghirri, A.; Chiesa, A.; Carretta, S.; Troiani, F.; Van Tol, J.; Hill, S.; Vitorica-Yrezabal, I.; Timco, G. A.; Winpenny, R. E. P.; Affronte, M.
abstract
Controlling and understanding transitions between molecular spin states allows selection of the most suitable ones for qubit encoding. Here we present a detailed investigation of single crystals of a polynuclear Cr8Zn molecular wheel using 241 GHz electron paramagnetic resonance (EPR) spectroscopy in high magnetic field. Continuous wave spectra are well reproduced by spin Hamiltonian calculations, which evidence that transitions in correspondence to a well-defined anticrossing involve mixed states with different total spin. We studied, by means of spin echo experiments, the temperature dependence of the dephasing time (T2) down to 1.35 K. These results are reproduced by considering both hyperfine and intermolecular dipolar interactions, evidencing that the dipolar contribution is completely suppressed at the lowest temperature. Overall, these results shed light on the effects of the decoherence mechanisms, whose understanding is crucial to exploit chemically engineered molecular states as a resource for quantum information processing.
2015
- Electroburning of few-layer graphene flakes, epitaxial graphene, and turbostratic graphene discs in air and under vacuum
[Articolo su rivista]
Candini, Andrea; Richter, Nils; Convertino, Domenica; Coletti, Camilla; Balestro, Franck; Wernsdorfer, Wolfgang; Kläui, Mathias; Affronte, Marco
abstract
Graphene-based electrodes are very promising for molecular electronics and spintronics. Here we report a systematic characterization
of the electroburning (EB) process, leading to the formation of nanometer-spaced gaps, on different types of few-layer
graphene (namely mechanically exfoliated graphene on SiO2, graphene epitaxially grown on the C-face of SiC and turbostratic
graphene discs deposited on SiO2) under air and vacuum conditions. The EB process is found to depend on both the graphene type
and on the ambient conditions. For the mechanically exfoliated graphene, performing EB under vacuum leads to a higher yield of
nanometer-gap formation than working in air. Conversely, for graphene on SiC the EB process is not successful under vacuum.
Finally, the EB is possible with turbostratic graphene discs only after the creation of a constriction in the sample using lithographic
patterning.
2015
- Low temperature magnetic properties and spin dynamics in single crystals of Cr8Zn antiferromagnetic molecular rings
[Articolo su rivista]
Adelnia, F.; Chiesa, A.; Bordignon, S.; Carretta, S.; Ghirri, A.; Candini, A.; Cervetti, C.; Evangelisti, M.; Affronte, M.; Sheikin, I.; Winpenny, R.; Timco, G.; Borsa, F.; Lascialfari, A.
abstract
2015
- Magnetic interplay between two different lanthanides in a tris-phthalocyaninato complex: a viable synthetic route and detailed investigation in the bulk and on the surface
[Articolo su rivista]
Lan, Yanhua; Klyatskaya, Svetlana; Ruben, Mario; Fuhr, Olaf; Wernsdorfer, Wolfgang; Candini, Andrea; Corradini, Valdis; LODI RIZZINI, Alberto; DEL PENNINO, Umberto; Troiani, Filippo; Joly, Loïc; Klar, David; Wende, Heiko; Affronte, Marco
abstract
Future applications of molecular units in quantum information technologies require a fine control at the
single molecule level. This includes the choice of each functional element, the intramolecular interaction
and the robustness of molecules when dispersed on a substrate. Keeping these goals in mind, we
designed and synthesized a heterometallic phthalocyaninato-complex including two different lanthanides
in each moiety, namely [PcDyPcTbPc*] (Pc being phthalocyanines; and Pc* being 2,3,9,10,16,17,23,24-
octahexyl-substituted phthalocyanines). Full magnetic characterization was performed down to the mK
temperature range on bulk microcrystals by means of AC susceptibility, DC magnetization (including
microSQUID) and specific heat measurements. A weak, yet sizeable, interaction between the two
lanthanides is clearly detected by different techniques, altering the magnetic behavior of the single
lanthanide as observed in the parent [LnPc2] complexes. Isolated [PcDyPcTbPc*] molecules dispersed on
HOPG and the Au surface by liquid phase deposition are proven to maintain their main chemical and
magnetic features by combined XPS, XAS and XMCD analysis and to lie with one Pc ligand flat to the
surface. Opening of a small but sizable hysteresis loop at 1.8 K is directly observed on both Tb and Dy
sites proving the retention of magnetization at the single molecule level.
2015
- Microstrip Resonators and Broadband Lines for X-band EPR Spectroscopy of Molecular Nanomagnets
[Articolo su rivista]
Ghirri, Alberto; Bonizzoni, Claudio; Righi, Mattia; Fedele, Federico; Timco, Grigore; Winpenny, Richard; Affronte, Marco
abstract
We present a practical setup to perform continuous-wave X-band
electron paramagnetic resonance spectroscopy by using planar microstrip lines and
general purpose instrumentation. We fabricated Ag/alumina and Nb/sapphire microstrip
resonators and transmission lines and compared their performance down to
2 K and under applied magnetic field. We used these devices to study single crystals
of molecular Cr3 nanomagnets. By means of X-band planar resonators we measured
angle-dependent spectra at fixed frequency, while broadband transmission lines
were used to measure continuous wave spectra with varying frequency in the range
2–25 GHz. The spectra acquired at low temperatures allowed to extract the essential
parameters of the low-lying energy levels of Cr3 and demonstrate that this method is
particularly suitable to study small crystals of molecular nanomagnets.
2015
- Observation of different charge transport regimes and large magnetoresistance in graphene oxide layers
[Articolo su rivista]
Vianelli, Anna; Candini, Andrea; Treossi, Emanuele; Palermo, Vincenzo; Affronte, Marco
abstract
We report a systematic study on charge transport properties of thermally reduced graphene
oxide (rGO) layers, from room temperature to 2 K and in presence of magnetic fields up to
7 T. The most conductive rGO sheets follow different transport regimes: at room temperature
they show an Arrhenius-like behavior. At lower temperature they exhibits a thermally
activated behavior with resistance R following a R = R0exp(T0/T)p law with p = 1/3, consistently
with 2D Mott Variable Range Hopping (VRH) transport mechanism. Below a given
temperature Tc, we observe a crossover from VHR to another regime, probably due to a
shortening of the characteristic lengths of the disordered 2D system. The temperature Tc
depends on the reduction grade of the rGO. Magnetoresistance DR/R of our rGO films shows
as well a crossover between positive and negative and below liquid He temperature DR/R
reaches values larger than 60%, surprisingly high for a – nominally – non magnetic
material.
2015
- Quantum Computation with Molecular Nanomagnets: Achievements, Challenges, and New Trends
[Capitolo/Saggio]
Ghirri, Alberto; Troiani, Filippo; Affronte, Marco
abstract
Molecular nanomagnets exhibit quanto-mechanical properties that can be nicely tailored at synthetic level: superposition and entanglement of quantum states can be created with molecular spins whose manipulation can be done in a timescale shorter than their decoherence time, if the molecular environment is controlled in a proper way. The challenge of quantum computation is to exploit the similarities between the coherent manipulation of molecular spins and algorithms used to process data and solve problems. In this chapter we shall firstly introduce basic concepts, stressing analogies between the physics and the chemistry of molecular nanomagnets and the science of computing. Then we shall review main achievements obtained in the first decade of this field and present challenges for the next future. In particular we shall focus on two emerging topics: quantum simulators and hybrid systems made by resonant cavities and molecular nanomagnets.
2015
- Single-Molecule Magnetism, Enhanced Magnetocaloric Effect, and Toroidal Magnetic Moments in a Family of Ln4 Squares
[Articolo su rivista]
Das, Chinmoy; Vaidya, Shefali; Gupta, Tulika; Frost, Jamie M.; Righi, Mattia; Brechin, Euan K.; Affronte, Marco; Rajaraman, Gopalan; Shanmugam, Maheswaran
abstract
Three cationic [Ln4] squares (Ln=lanthanide) were isolated as single crystals and their structures solved as [Dy4(μ4-OH)(HL)(H2L)3(H2O)4]Cl2·(CH3OH)4·(H2O)8 (1), [Tb4(μ4-OH)(HL)(H2L)3(MeOH)4]Cl2·(CH3OH)4·(H2O)4 (2) and [Gd4(μ4-OH)(HL)(H2L)3(H2O)2(MeOH)2]Br2·(CH3OH)4·(H2O)3 (3). The structures are described as hydroxo-centered squares of lanthanide ions, with each edge of the square bridged by a doubly deprotonated H2L2- ligand. Alternating current magnetic susceptibility measurements show frequency-dependent out-of-phase signals with two different thermally assisted relaxation processes for 1, whereas no maxima in χM" appears above 2.0 K for complex 2. For 1, the estimated effective energy barrier for these two relaxation processes is 29 and 100 K. Detailed ab initio studies reveal that complex 1 possesses a toroidal magnetic moment. The ab initio calculated anisotropies of the metal ions in complex 1 were employed to simulate the magnetic susceptibility by using the Lines model (POLY-ANISO) and this procedure yields J1=+0.01 and J2=-0.01 cm-1 for 1 as the two distinct exchange interactions between the DyIII ions. Similar parameters are also obtained for complex 1 (and 2) from specific heat measurements. A very weak antiferromagnetic super-exchange interaction (J1=-0.043 cm-1 and g=1.99) is observed between the metal centers in 3. The magnetocaloric effect (MCE) was estimated by using field-dependent magnetization and temperature-dependent heat-capacity measurements. An excellent agreement is found for the -ΔSm values extracted from these two measurements for all three complexes. As expected, 3 shows the largest -ΔSm variation (23 J Kg-1 K-1) among the three complexes. The negligible magnetic anisotropy of Gd indeed ensures near degeneracy in the (2S+1) ground state microstates, and the weak super-exchange interaction facilitates dense population of low-lying excited states, all of which are likely to contribute to the MCE, making complex 3 an attractive candidate for cryogenic refrigeration.
2015
- Trioctahedral Fe-rich micas: Relationships between magnetic behavior and crystal chemistry
[Articolo su rivista]
Brigatti, Maria Franca; Affronte, Marco; Elmi, Chiara; Malferrari, Daniele; Laurora, Angela
abstract
Six Fe-bearing mica samples with different Fe ordering, Fe2+/(Fe2++Fe3+) ratio, octahedral, and tetrahedral composition were studied. Four micas belong to the phlogopite-annite join (space group C2/m), two are Mg-rich annite and two are Fe-rich phlogopite, one is a tetra-ferriphlogopite (space group C2/m) and one is Li-rich siderophyllite (space group C2). Thus these samples had a different environment around the Fe cations and layer symmetry. These micas were characterized by chemical analyses, single-crystal X-ray diffraction, X-ray absorption spectroscopy, and magnetic measurements. In samples with Fe mostly in octahedral coordination, dominant magnetic interactions among Fe atoms are ferromagnetic, which reach a maximum at higher Fe2+/(Fe2++Fe3+) ratios. Samples with higher Fe2+/(Fe2++Fe3+) ratio are also characterized by higher values of the Curie-Weiss theta constant. Where Fe2+/(Fe2++Fe3+) ratios decrease, theta values also decrease. The Fe3+-rich phlogopite shows predominant Fe3+ in tetrahedral coordination and shows anti-ferromagnetic interactions with a negative value of the Curie-Weiss theta constant (i.e., theta = 25 K). Fe ordering in octahedral trans- and in one of the two cis-sites accounts for a greater theta value in Li-rich siderophyllite when compared to other samples showing similar octahedral Fe content. Our data suggest that Fe3+ cations and other non-ferromagnetic cations hinder long range magnetic ordering. This observation may produce for the different role of octahedral Fe magnetic interactions that can in principle develop along long Fe-rich octahedral chains, when compared to tetrahedral-octahedral interactions that are confined within the layer by the non-ferromagnetic cations of the interlayer. Spin glass behavior is indicated by the dependency of the temperature to produce maxima in the susceptibility curve. These maxima are related to the frequency of the applied AC magnetic field.
2015
- YBa2Cu3O7 microwave resonators for strong collective coupling with spin ensembles
[Articolo su rivista]
Ghirri, Alberto; Bonizzoni, Claudio; Gerace, D.; Sanna, S.; Cassinese, A.; Affronte, Marco
abstract
Coplanar microwave resonators made of 330 nm-thick superconducting YBa2Cu3O7 have been
realized and characterized in a wide temperature (T, 2–100 K) and magnetic field (B, 0–7 T) range.
The quality factor (QL) exceeds 104 below 55K and it slightly decreases for increasing fields,
remaining 90% of QLðB ¼ 0Þ for B¼7 T and T¼2K. These features allow the coherent coupling
of resonant photons with a spin ensemble at finite temperature and magnetic field. To demonstrate
this, collective strong coupling was achieved by using di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium
organic radical placed at the magnetic antinode of the fundamental mode: the in-plane magnetic
field is used to tune the spin frequency gap splitting across the single-mode cavity resonance
at 7.75 GHz, where clear anticrossings are observed with a splitting as large as 82 MHz at
T¼2K. The spin-cavity collective coupling rate is shown to scale as the square root of the number
of active spins in the ensemble.
2014
- A Detailed Study of the Magnetism of Chiral {Cr7M} Rings: An Investigation into Parametrization and Transferability of Parameters
[Articolo su rivista]
Garlatti, Elena; Albring, Morten A.; Baker, Michael L.; Docherty, Rebecca J.; Mutka, Hannu; Guidi, Tatiana; Garcia Sakai, Victoria; Whitehead, George F. S.; Pritchard, Robin G.; Timco, Grigore A.; Tuna, Floriana; Amoretti, Giuseppe; Carretta, Stefano; Santini, Paolo; Lorusso, Giulia; Affronte, Marco; Mcinnes, Eric J. L.; Collison, David; Winpenny, Richard E. P.
abstract
Compounds of general formula [Cr7MF3(Etglu)-
(O2CtBu)15(Phpy)] [H5Etglu = N-ethyl-D-glucamine; Phpy = 4-phenylpyridine;
M = Zn (1), Mn (2), Ni (3)] have been prepared. The structures
contain an irregular octagon of metal sites formed around the pentadeprotonated
Etglu5− ligand; the chirality of N-ethyl-D-glucamine is retained
in the final product. The seven CrIII sites have a range of coordination
environments, and the divalent metal site is crystallographically identified
and has a Phpy ligand attached to it. By using complementary experimental
techniques, including magnetization and specific heat measurements,
inelastic neutron scattering, and electron paramagnetic resonance spectroscopy,
we have investigated the magnetic features of this family of {Cr7M}
rings. Microscopic parameters of the spin Hamiltonian have been
determined as a result of best fits of the different experimental data,
allowing a direct comparison with corresponding parameters found in the
parent compounds. We examine whether these parameters can be transferred between compounds and compare them with those
of an earlier family of heterometallic rings.
2014
- DFT Study of the Cr8 Molecular Magnet Within Chain-Model Approximations
[Relazione in Atti di Convegno]
Bellini, Valerio; Daria, M. Tomecka; Brzostowski, Bartosz; Wojciechowski, Michał; Troiani, Filippo; Manghi, Franca; Affronte, Marco
abstract
We present a density functional theory (DFT) study of the
electronic and magnetic properties of the Cr8 molecular ring. The allelectron
linearized augmented plane wave method (LAPW) implemented
in the Wien2k package and pseudopotential method implemented in
SIESTA package are used to calculate the electronic states, exchange
coupling parameters of an infinite chain model system of Cr8.We demonstrate
how, under opportune modifications to the ring cycle structure,
different one-dimensional chain models can be devised, with the capability
of mimicking with good approximation the electronic and magnetic
properties of the original Cr8 molecule. Such models offer an unique
opportunity, in virtue of the reduced computational effort, to carry out
extensive investigations of a whole set of molecules belonging to the Crbased
molecular rings family.
2014
- Ferromagnetic Exchange Coupling between Fe Phthalocyanine and Ni(111) Surface Mediated by the Extended States of Graphene
[Articolo su rivista]
Candini, Andrea; Bellini, Valerio; Klar, David; Corradini, Valdis; Biagi, Roberto; DE RENZI, Valentina; Kummer, Kurt; Brookes, Nicholas B.; DEL PENNINO, Umberto; Wende, Heiko; Affronte, Marco
abstract
The interface spin coupling mechanism is studied in a hybrid structure
made of Fe phthalocyanine molecules sublimed in ultrahigh vacuum on graphene grown
on the magnetic substrate Ni(111). By using synchrotron X-ray magnetic circular
dichroism, the field-dependent magnetization of the isolated FePc molecules and of the
Ni substrate has been measured at low temperature (8 K). Along with density functional
theory calculations, the role of the graphene interlayer in transmitting the magnetic
coupling is addressed. Both experiments and theory show a ferromagnetic coupling
between the molecules and the substrate which is weakened by the insertion of graphene.
DFT calculations indicate that the key role is played by the π orbitals of graphene, which
hybridize with the underlying magnetic Ni, giving rise to a sizable spin polarized
continuum at the molecular interface. The resulting overlap with the Fe orbitals favors a
direct coupling of ferromagnetic nature, as evidenced by our spin density distribution plots.
2014
- Field-regulated switching of the magnetization of Co-porphyrin on graphene
[Articolo su rivista]
Klar, D.; Bhandary, S.; Candini, Andrea; Joly, L.; Ohresser, P.; Klyatskaya, S.; Schleberger, M.; Ruben, M.; Affronte, Marco; Eriksson, O.; Sanyal, B.; Wende, H.
abstract
Differentmagnetic couplingmechanisms have been identified for a fewmonolayers of Co-porphyrin molecules
deposited on a graphene-covered Ni(111) single crystal. A relatively strong antiferromagnetic coupling of the
first molecular layer via graphene to the Ni crystal in comparison to a weaker intermolecular coupling gives
rise to a complex field-dependent response of this hybrid system. By continuously increasing the magnetic field
strength, the net magnetization of the molecular system switches from antiparallel to parallel to the field direction
at 2.5 T. Utilizing x-ray absorption spectroscopy and x-ray magnetic circular dichroism, the element-specific
magnetization and field dependence was probed. The nature of the magnetic couplings is identified by means of
density functional theory and orbital-dependent susceptibilities.
2014
- Octanuclear [Ni(II)₄Ln(III)₄] complexes. Synthesis, crystal structures and magnetocaloric properties
[Articolo su rivista]
Pasatoiu, Traian D; Ghirri, Alberto; Madalan, Augustin M; Affronte, Marco; Andruh, Marius
abstract
Two original heterooctanuclear [Ni(II)4Ln(III)4] complexes (Ln(III) = Sm(III), Gd(III)) have been obtained starting from the [Ni(II)(valpn)(H2O)2] mononuclear precursor [H2valpn = 1,3-propanediylbis(2-iminomethylene-6-methoxy-phenol)] and the corresponding lanthanide nitrates, in the presence of azide anions, through slow capture of atmospheric CO2. Three weak and competitive exchange interactions, J(GdGd), J(GdNi), J(NiNi), make the ground state of this magnetic system degenerate at cryogenic temperature and zero field. This, along with the high spin of Gd(III), lead to a significant magnetocaloric effect spread in the temperature range 1 to 20 K (ΔSm[0-7 T, 3.5 K] = 19 J kg(-1) K(-1)).
2014
- Potentialities of Molecular Nanomagnets for Information TechnologiesMolecular Magnets
[Capitolo/Saggio]
Affronte, Marco; Filippo, Troiani
abstract
The possibility of tailoring their functionalities at the molecular scale
makes Molecular nanoMagnets interesting for applications in Information Technolo-
gies where the race for extreme miniaturization will soon lead at requiring com-
ponents of few nanometers in size. Properties like the magnetic bistability or the
switchability by external stimuli actually allow to mimic, at the molecular scale, ba-
sic operations commonly used in computers while embedding magnetic molecules in
suitable electronic circuits allows the fabrication of novel spintronic devices. Even
more challenging is the control and the exploitation of quantum properties in molec-
ular spin clusters that may allow the encoding quantum information with molecules.
These concepts are substantiated by many achievements obtained in the recent years
and presented in this chapter along with some perspectives and next challenges for
the future.
2014
- Surface Investigation on Gd4M8(M = Zn, Ni) Single Molecule Coolers
[Articolo su rivista]
Corradini, Valdis; Ghirri, Alberto; Candini, Andrea; Biagi, Roberto; DEL PENNINO, Umberto; DE RENZI, Valentina; Dotti, Gianluca; Otero, Edwige; Hooper, Thomas N.; Inglis, Ross; Brechin, Euan K.; Affronte, Marco
abstract
Proving the preservation of functionality at the molecular scale is still a major challenge, both for the choice of suitable derivatives and protocols and for the employed experimental tools and methods. By using a combined scanning probe (AFM and STM) and spectroscopic methodology (XPS, XAS and XMCD) we show that Gd4M8 derivatives (with M=Zn2+, Ni2+) are robust molecular units which preserve their electronic, magnetic and thermodynamic properties when deposited on a metallic surface. Namely we measured entropy variation ΔS=[S(6T)-S(0T)] exceeding 8R (20 J Kg-1 K-1) at 4K in isolated Gd4Ni8 molecules dispersed on Au(111) surface and show a viable route to exploit large magnetocaloric effect at single molecule level.
2014
- Synthesis and magnetothermal properties of a ferromagnetically coupled NiII–GdIII–NiII cluster
[Articolo su rivista]
Apoorva, Upadhyay; Navatha, Komatireddy; GHIRRI, Alberto; Floriana, Tuna; Stuart K., Langley; Anant K., Srivastava; E., Carolina Sañudo; Boujemaa, Moubaraki; Keith S., Murray; Eric J. L., McInnes; AFFRONTE, Marco; Maheswaran, Shanmugam
abstract
A linear trimeric cluster of molecular formula [Ni2Gd(L−)6](NO3) (1) (L− = (C14H12NO2) has been isolated with its structure determined via single crystal X-ray diffraction. Magnetic susceptibility measurements of 1 show that the nickel and gadolinium ions are coupled ferromagnetically, with a ground total spin state (S) of 11/2. Best fit spin Hamiltonian parameters obtained for 1 are J1(Ni–Gd) = +0.54 cm−1, g = 2.01. EPR measurements confirm a low magnetic anisotropy (D = −0.135 cm−1) for 1. Heat capacity determination of the magnetocaloric effect (MCE) parameters for 1 shows that the change in magnetic entropy (−ΔSm) achieves a maximum of 13.74 J kg−1 K−1 at 4.0 K, with the ferromagnetic coupling giving a rapid change in low applied fields, confirming the potential of Gd molecular derivatives as coolants at liquid helium temperature.
2013
- Antiferromagnetic coupling of TbPc2 molecules to ultrathin Ni and Co films
[Articolo su rivista]
David, Klar; Svetlana, Klyatskaya; Andrea, Candini; Bernhard, Krumme; Kurt, Kummer; Philippe, Ohresser; Valdis, Corradini; DE RENZI, Valentina; Biagi, Roberto; Loic, Joly; Jean Paul, Kappler; DEL PENNINO, Umberto; Affronte, Marco; Heiko, Wende; Mario, Ruben
abstract
The magnetic and electronic properties of single-molecule magnets are studied by X-ray absorption spectroscopy and X-ray
magnetic circular dichroism. We study the magnetic coupling of ultrathin Co and Ni films that are epitaxially grown onto a Cu(100)
substrate, to an in situ deposited submonolayer of TbPc2
molecules. Because of the element specificity of the X-ray absorption
spectroscopy we are able to individually determine the field dependence of the magnetization of the Tb ions and the Ni or Co film.
On both substrates the TbPc2 moleculescouple antiferromagnetically to the ferromagnetic films, which is possibly due to a superexchange interaction via the phthalocyanine ligand that contacts the magnetic surface.
2013
- Inhomogeneous magnetism in the doped kagome lattice of LaCuO2.66
[Articolo su rivista]
M. H., Julien; V., Simonet; B., Canals; R., Ballou; A. K., Hassan; Affronte, Marco; V. O., Garlea; C., Darie; P., Bordet
abstract
The hole-doped kagome lattice of Cu2+ ions in LaCuO2.66 was investigated by nuclear quadrupole
resonance, electron spin resonance, electrical resistivity, bulkmagnetization, and specific-heat measurements. For
temperatures above∼180 K, the spin and charge properties showan activated behavior suggestive of a narrow-gap
semiconductor. At lower temperatures, the results indicate an insulating ground state which may or may not be
charge ordered. While the frustrated spins in remaining patches of the original kagome lattice might not be
directly detected here, the observation of coexisting nonmagnetic sites, free spins, and frozen moments reveals
an intrinsically inhomogeneous magnetism. Numerical simulations of a 1
3 -diluted kagome lattice rationalize this
magnetic state in terms of a heterogeneous distribution of cluster sizes and morphologies near the site-percolation
threshold.
2013
- Magnetic Cooling at a Single Molecule Level: a Spectroscopic Investigation of Isolated Molecules on a Surface
[Articolo su rivista]
Valdis, Corradini; Alberto, Ghirri; Andrea, Candini; Biagi, Roberto; DEL PENNINO, Umberto; Gianluca, Dotti; Edwige, Otero; Fadi, Choueikani; Robin J., Blagg; Eric J. L., Mcinnes; Affronte, Marco
abstract
A sub-monolayer distribution of isolated molecular Fe-14(bta)(6) nanomagnets is deposited intact on a Au(111) surface and investigated by X-ray magnetic circular dichroism spectroscopy. The entropy variation with respect to the applied magnetic field is extracted from the magnetization curves and evidences high magnetocaloric values at the single molecule level.
2013
- Studies of Hybrid Organic-Inorganic [2] and [3] Rotaxanes Bound to Au Surfaces
[Articolo su rivista]
H., Rath; G. A., Timco; V., Corradini; A., Ghirri; DEL PENNINO, Umberto; A. F., Lisardo; R. G., Pritchard; AFFRONTE, Marco; R. E. P., Winpenny
abstract
Hybrid organic–inorganic [2]- and [3]rotaxanes have been synthesised,
and their ability to bind to Au surfaces studied; the length of
the tethering group is found to control how the supramolecular
assembly binds to the surface and we find that [2]rotaxanes show
improved stability over previous studies of simple inorganic rings.
2012
- Controlled Positioning of Nanoparticles on Graphene by Non-Invasive AFM lithography
[Articolo su rivista]
E., Bellido; I., Ojea Jiménez; A., Ghirri; C., Alvino; A., Candini; V., Puntes; Affronte, Marco; N., Domingo; D., Ruiz Molina
abstract
Atomic force microscopy is shown to be an excellent lithographic
technique to directly deposit nanoparticles on graphene by capillary transport
without any previous functionalization of neither the nanoparticles nor the
graphene surface while preserving its integrity and conductivity properties.
Moreover this technique allows for (sub)micrometric control on the positioning
thanks to a new three-step protocol that has been designed with this aim. With
this methodology the exact target coordinates are registered by scanning the tip
over the predetermined area previous to its coating with the ink and deposition.
As a proof-of-concept, this strategy has successfully allowed the controlled
deposition of few nanoparticles on 1 μm2 preselected sites of a graphene surface
with high accuracy.
2012
- Dodecanuclear [CuII6GdIII6] Nanoclusters as Magnetic Refrigerants.
[Articolo su rivista]
A. S., Dinca; A., Ghirri; A. M., Madalan; Affronte, Marco; M., Andruh
abstract
A novel dodecanuclear complex, [{(HL)-(L)(DMF)CuIIGdIII(DMF)(H2O)}6]·6DMF (1; DMF =N,N-dimethylformamide), has been obtained using theligand resulting from the condensation of 3-formylsalicylicacid with hydroxylamine (H3L). The exchange interactionbetween the phenoxo-bridged CuII and GdIII ions is weakferromagnetic (J = +1.01 cm−1). The combination of ahigh-spin ground state with small anisotropy leads to asignificant magnetocaloric effect [−ΔSm(0−7 T) = 23.5 JK g−1 K−1 at ∼2 K].
2012
- Magnetic and entanglement properties of molecular Cr2nCu2 heterometallic spin rings
[Articolo su rivista]
Lorusso, Giulia; V., Corradini; A., Ghirri; Biagi, Roberto; DEL PENNINO, Umberto; Siloi, Ilaria; F., Troiani; G., Timco; R. E. P., Winpenny; Affronte, Marco
abstract
We investigate the low-temperature magnetic and entanglement properties of a series of molecular Cr2nCu2
heterometallic spin rings (withn=4,5,6). These are cyclic spin systems, consisting of two Cu2+(s=1/2) ions, coupled by two antiferromagnetic segments of nCr3+(s=3/2) ions. Thermodynamic measurements (magnetization, susceptibility, and specific heat) allow us to determine the total spin of the ground state and to estimate the spin-Hamiltonian parameters related to magnetic anisotropy. X-ray spectroscopies (XAS and
XMCD) are used to probe the local magnetization of the Cr and Cu ions, and provide results that are consistent with the bulk magnetization data. We finally investigate the relation between heterometallicity and entanglement in these prototypical spin systems. In particular, we focus on the spatial modulation of entanglement induced by the Cu defect spins and on the long-distance entanglement between them induced by the two Cr chains.
2012
- Magnetic Anisotropy of Cr7Ni Spin Clusters on Surfaces
[Articolo su rivista]
Valdis, Corradini; Alberto, Ghirri; Elena, Garlatti; Biagi, Roberto; DE RENZI, Valentina; DEL PENNINO, Umberto; Valerio, Bellini; Stefano, Carretta; Paolo, Santini; Grigore, Timco; Richard E. P., Winpenny; Affronte, Marco
abstract
The problem of the experimental and theoretical determination of magnetic anisotropy in isolated molecular spin clusters is addressed here. To this end, the case of molecular Cr 7 Ni rings sublimated in ultrahigh vacuum conditions and assembled in an ordered fashion on Au(111) surface is addressed and
investigated using X-ray magnetic dichroism (XMCD) and theoretical calculations. Fixing the experimental conditions at a temperature T = 8 K and a magnetic fi eld of 5 T, the angular-dependence of the dichroic signal reveals an easy-axis anisotropy for the Ni magnetization along the direction perpendicular to the ring while the magnetization of the whole Cr 7 Ni molecule is preferentially aligned within the ring plane.
2012
- Magnetic behavior of trioctahedral micas with different octahedral Fe ordering
[Articolo su rivista]
Pini, Stefano; Brigatti, Maria Franca; Affronte, Marco; Malferrari, Daniele; A., Marcelli
abstract
This contribution is finalized at the discussion of the magnetic structure of two samples, belonging to phlogopite–annite [sample TK, chemical composition IV(Si2.76Al1.24) VI(Al0.64Mg0.72 Fe1.452+ Mn0.03Ti0.15) (K0.96Na0.05) O10.67 (OH)1.31 Cl0.02] and polylithionite–siderophyllite joints [sample PPB, chemical composition IV(Si3.14Al0.86)VI(Al0.75Mg0.01 Fe1.032+ Fe1.033+ Mn0.01Ti0.01Li1.09) (K0.99Na0.01) O10.00 (OH)0.65F1.35]. Samples differ for Fe ordering in octahedral sites, Fe2+/(Fe2+ + Fe3+) ratio, octahedral composition, defining a different environment around Fe cations, and layer symmetry. Spin-glass behavior was detected for both samples, as evidenced by the dependency of the temperature giving the peak in the susceptibility curve from the frequency of the applied alternating current magnetic field. The crystal chemical features are associated to the different temperature at which the maximum in magnetic susceptibility is observed: 6 K in TK, where Fe is disordered in all octahedral sites, and 8 K in PPB sample, showing a smaller and more regular coordination polyhedron for Fe, which is ordered in the trans-site and in one of the two cis-sites.
2011
- Characterization of a new cobalt precursor for focused beam deposition of magnetic nanostructures
[Articolo su rivista]
Gazzadi, gian carlo; J. J. L., Mulders; P., Trompenaars; Ghirri, Alberto; Rota, Alberto; Affronte, Marco; Frabboni, Stefano
abstract
The electrical and magnetic properties of nanowires deposited from cobalt tricarbonyl nitrosyl (Co(CO)(3)NO) precursor by focused electron beam- and focused ion beam-induced deposition (FEBID and FIBID) have been investigated. As-deposited nanowires have similar Co content, around 50-55 at.%, but different electrical behaviour: FEBID nanowire is highly resistive (6.3 m Omega cm at RT) and non-metallic at low T, while the FIBID one has much lower resistivity (189 mu Omega cm at RT) and it is metallic. The magnetic properties, tested with magnetoresistance measurements, reveal a non-magnetic behaviour for both nanowires. After 400 degrees C annealing in vacuum FEBID wire is much less resistive (62 mu Omega cm at RT) and recovers the metallic behaviour at low T, and both FEBID and FIBID wires display ferromagnetic behaviour. Structural analysis by low energy-scanning transmission electron microscopy (LE-STEM) suggests that coarsening and interconnection of the Co nanograins are responsible for the improvement in electrical and magnetic properties.
2011
- Chemical control of spin propagation between heterometallic rings
[Articolo su rivista]
T. B., Faust; BELLINI, Valerio; CANDINI, Andrea; S., Carretta; L., Carthy; B. J., Coe; D., Collison; R. J., Docherty; J., Kenyon; LORUSSO, Giulia; J., Machin; E. J. L., McInnes; C. A., Muryn; R. G., Pritchard; S. J., Teat; G. A., Timco; F., Tuna; G. F., Whitehead; W., Wernsdorfer; AFFRONTE, Marco; R. E. P., Winpenny
abstract
We present a synthetic, structural,theoretical, and spectroscopicstudy of a family of heterometallic ringdimers which have the formula[{Cr7NiF3ACHTUNGTRENUNG(Etglu)ACHTUNGTRENUNG(O2CtBu)15}2ACHTUNGTRENUNG(NLN)],in which Etglu is the pentadeprotonatedform of the sugar N-ethyl-d-glucamine,and NLN is an aromatic bridgingdiACHTUNGTRENUNGimine ligand. By varying NLN we areable to adjust the strength of the interactionbetween rings with the aim ofunderstanding how to tune our systemto achieve weak magnetic communicationbetween the spins, a prerequisitefor quantum entanglement. Micro-SQUID and EPR data reveal that themagnetic coupling between rings ispartly related to the through-bond distancebetween the spin centers, butalso depends on spin-polarizationmechanisms and torsion angles betweenaromatic rings. Density functionaltheory (DFT) calculations allow usto make predictions of how such chemicallyvariable parameters could beused to tune very precisely the interactionin such systems. For possible applicationsin quantum information processingand molecular spintronics, suchprecise control is essential.
2011
- Focused Electron Beam Deposition of Nanowires from Cobalt Tricarbonyl Nitrosyl (Co(CO)(3)NO) Precursor
[Articolo su rivista]
Gazzadi, gian carlo; Mulders, H.; Trompenaars, P.; Ghirri, A.; Affronte, Marco; Grillo, V.; Frabboni, Stefano
abstract
Nanowires deposited by focused electron beam-induced deposition (FEBID) of cobalt tricarbonyl nitrosyl (Co(CO)(3)NO) precursor have been thoroughly characterized from an electrical, magnetic, and structural point of view. Deposit composition and deposition yield have been studied as a function of beam energy and current. Atomic concentrations are weakly dependent on beam parameters and have average values of 49 at % for Co, 27 at % for O, 14 at % for N, and 10 at % for C. Deposition yield decreases as the beam energy increases, and strong enhancement (67x) is observed for deposition at 130 degrees C substrate temperature. FEBID nanowires are highly resistive (rho = 6.3 m Omega cm at RT), and resistivity increases as T decreases with a power law behavior typical of metal-insulator (M-I) nanogranular systems. Nonmagnetic behavior is revealed by magnetoresistance (MR) measurements. After 400 degrees C vacuum annealing, conductivity of the nanowire is greatly improved (rho = 62 mu Omega cm at RT), and a metallic-like resistivity is fully recovered. MR angular plots display a (cos theta)(2) dependence typical of anisotropic MR (AMR) in ferromagnets, with AMR values of 0.85%. Co concentration in the deposit is not significantly increased though. TEM structural analysis reveals that before annealing the deposit has a CoO fcc structural phase with nanograins size around 1 nm. After annealing, a new Co hcp phase shows up beside CoO fcc, and coarsening (10 - 15 nm) and interconnection of the Co nanograins are observed, providing the conditions for ferromagnetism and metallic electrical transport.
2011
- Graphene Spintronic Devices with Molecular Nanomagnets.
[Articolo su rivista]
A., Candini; S., Klyatskaya; M., Ruben; W., Wernsdorfer; Affronte, Marco
abstract
The possibility to graft nano-objects directly on its surface makesgraphene particularly appealing for device and sensing applications. Here we reportthe design and the realization of a novel device made by a graphene nanoconstrictiondecorated with TbPc2 magnetic molecules (Pc = phthalocyananine), to electricallydetect the magnetization reversal of the molecules in proximity with graphene. Amagnetoconductivity signal as high as 20% is found for the spin reversal, revealing theuniaxial magnetic anisotropy of the TbPc2 quantum magnets. These results depict thebehavior of multiple-field-effect nanotransistors with sensitivity at the single-molecule level.
2011
- Hysteresis loops of magnetoconductance in graphene devices
[Articolo su rivista]
A., Candini; C., Alvino; W., Wernsdorfer; Affronte, Marco
abstract
We report very low-temperature magnetoconductance G measurements on graphene devices with themagnetic field H applied parallel to the carbon sheet. The G(H) signal depends on the gate voltage Vgand its sign is related to the universal conductance fluctuations. When the magnetic field is swept at fast rates,G displays hysteresis loops evident for different sizes and at different transport regimes of the devices. Weattribute this to the magnetization reversal of paramagnetic centers in the graphene layer, which might originatefrom defects in our devices.
2011
- Molecular Spins for Quantum Information Technologies
[Articolo su rivista]
F., Troiani; Affronte, Marco
abstract
Technological challenges for quantum information technologies lead us to consider aspectsof molecular magnetism in a radically new perspective. The design of new derivatives and recentexperimental results on molecular nanomagnets are covered in this tutorial review through thekeyhole of basic concepts of quantum information, such as the control of decoherence andentanglement at the (supra-)molecular level.
2011
- Oxo-centered carboxylate-bridged trinuclear complexes deposited on Au(111) by a mass-selective electrospray.
[Articolo su rivista]
Corradini, Valdis; C., Cervetti; Ghirri, Alberto; Biagi, Roberto; DEL PENNINO, Umberto; G. A., Timco; R. E. P., Winpenny; Affronte, Marco
abstract
We developed an apparatus for nondestructive in vacuum deposition of mass-selected fragile Cr based metal trinuclear complexes, by modifying a commercial Mass Spectrometer containing an electrospray ionization source. Starting from a solution, this system creates a beam of ionized molecules which is then transferred into an evacuated region where the molecules can be mass selected before deposition. To verify the system efficiency, we deposited sub monolayers of oxo-centered carboxylate-bridged trinuclear complexes (Cr3 and Cr2Ni) on Au(111) surface. By XPS and STM we determined the deposited molecule stoichiometry and the surface coverage. The results show that this apparatus is works well for the in vacuum deposition of molecular nanomagnets and, thanks to its reduced dimensions, it is portable.
2011
- Propagation of spin information at supra-molecular scale through hetero-aromatic linkers
[Articolo su rivista]
V., Bellini; G., Lorusso; A., Candini; T. B., Faust; G. A., Timco; W., Wernsdorfer; R. E. P., Winpenny; Affronte, Marco
abstract
We report an in-depth study on how spin information propagates at supramolecular scale through afamily of heteroaromatic linkers. By density-functional theory calculations, we rationalize the behavior ofa series of Cr7Ni dimers for which we are able to systematically change the aromatic linker thus tuning thestrength of the magnetic interaction, as experimentally shown by low temperature micro-SQUID andspecific heat measurements. We also predict a cos2 dependence of the magnetic coupling on the twistingangle between the aromatic cycles in bicyclic linkers, a mechanism parallel to charge transport on similarsystems [L. Venkataraman et al., Nature (London) 442, 904 (2006)].
2011
- Self-assembled monolayer of Cr7Ni molecular nanomagnets by sublimation.
[Articolo su rivista]
Ghirri, Alberto; Corradini, Valdis; Bellini, Valerio; Biagi, Roberto; DEL PENNINO, Umberto; DE RENZI, Valentina; Julio C., Cezar; Christopher A., Muryn; Grigore A., Timco; Richard E. P., Winpenny; Affronte, Marco
abstract
We show, by complementary spectroscopic and STM analysis, that Cr7Ni derivativesare suitable to be sublimed in UHV conditions. Cr7Ni-bu weakly bonds to gold surface and can diffuserelatively freely on it, forming monolayers with hexagonal 2D packing. Conversely, by adding afunctional thiol group to the central dibutylamine, a covalent bond between the molecule andsurface gold adatoms is promoted, leading to a strong molecular grafting and the formation of adisordered monolayer. These two examples demonstrate the possibility to control the assembly of alarge molecular complex, as rationalized by DFT calculations that establish different energy scales inthe deposition processes. Moreover, low-temperature XMCD sprectra show that the magneticfeatures of Cr7Ni rings deposited in UHV on gold remain unchanged with respect to those of thecorresponding bulk sample.
2010
- A density-functional study of heterometallic Cr-based molecular rings
[Articolo su rivista]
Bellini, V.; Affronte, M.
abstract
We present a density-functional theoretical investigation of the electronic and magnetic properties of octametallic Cr-based molecular antiferromagnetic rings. The presence of the divalent magnetic ion M unbalances the charge and the spin of the parent Cr8 ring, leading to a finite total spin in the molecules. Results are presented for Cr8, i.e., [Cr8F 8(O2CH)16] (1), and for Cr7M rings belonging to two different derivatives, i.e., [Me2NH 2][Cr7MF8(O2CH)16], with M = Ni, Mn, Fe, and Cu, and Me=CH3 (2, "green" derivative), and [Cr7 NiF3(C6H10NO 5)(O2CH)15 (H2O)] (3, "purple" derivative). Exchange interaction parameters have been extracted from broken-symmetry calculations and compared with the available experiments; in agreement with them, we find that exchange parameters are rather similar in the two derivatives, although somewhat larger in the "purple" derivative. The analysis of the electronic properties shows some differences depending on M, in particular in the size of the highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO-LUMO) gaps. For all the "green" rings we observe that the HOMOs are localized on the divalent ion site, while the HOMOs for the "purple" Cr7Ni have a more delocalized nature; LUMOs, instead, are, except for "green" Cr7Cu, localized on the Cr atoms opposite to the M site. We discuss how these findings may show up in terms of an asymmetric I-V curve in molecular junctions working in the sequential tunneling regime, or help in discerning the orientation of the molecules with respect to a surface, in scanning tunneling experiments. © 2010 American Chemical Society.
2010
- Deposition of Functionalized Cr7Ni “Molecular Rings on Graphite from the Liquid Phase
[Articolo su rivista]
Ghirri, Alberto; V., Corradini; Cervetti, Christian; Candini, Andrea; DEL PENNINO, Umberto; G., Timco; R. J., Pritchard; C. A., Muryn; R. E. P., Winpenny; Affronte, Marco
abstract
Graphite is a clean substrate and its nanostructures hold great potentialities for applications.Anchoring large molecules on graphite represents a challenge for several reasons thatessentially rise from the planar bonds of the packed honeycomb structure of carbon. Here wereport a systematic investigation by AFM and XPS on different derivatives of molecularCr7Ni rings deposited on highly oriented pyrolytic graphite (HOPG). Cr7Ni is emerging as aprototipical example of molecular antiferromagnet on which quantum phenomena andcoherence have been demonstrated. For the deposition of Cr7Ni on HOPG, we essentiallyadopt two strategies: in the first instance, Cr7Ni rings have been functionalized with extendedalkyl/benzene terminations while, in the second case, we firstly deposit a self-assembledmonolayer of alkyl chains with sulfonate terminations and then we use a cationic Cr7Niderivative. In both cases the electronic bond with the carbon surface is soft but the two-stepprocedure results efficient, albeit indirect, in sticking molecular Cr7Ni on HOPG. Thesestrategies can be easily extended to deposit other complex molecular aggregates on graphiteby liquid phase.
2010
- Entanglement in supramolecular spin systems of two weakly coupled antiferromagnetic rings (purple Cr7Ni)
[Articolo su rivista]
A., Candini; G., Lorusso; F., Troiani; A., Ghirri; S., Carretta; P., Santini; G., Amoretti; C., Muryn; F., Tuna; G., Timco; E. J. L., Mcinnes; R. E. P., Winpenny; W., Wernsdorfer; Affronte, Marco
abstract
We characterize supramolecular magnetic structures, consisting of two weakly coupled antiferromagneticrings, by low-temperature specific heat, susceptibility, magnetization and electron paramagneticresonance measurements. Intra- and inter-ring interactions are modeled through a microscopic spin-Hamiltonian approach that reproduces all the experimental data quantitatively and legitimates the use ofan effective two-qubit picture. Spin entanglement between the rings is experimentally demonstratedthrough magnetic susceptibility below 50 mK and theoretically quantified by the concurrence.
2010
- Grafting molecular Cr7Ni rings on a gold surface
[Articolo su rivista]
Corradini, Valdis; Ghirri, Alberto; Del Pennino, Umberto; Biagi, Roberto; Victoria A., Milway; Grigore, Timco; Floriana, Tuna; Richard E. P., Winpenny; Affronte, Marco
abstract
Molecular {Cr7Ni} rings have shown several ideal features for the observation of quantum phenomena and they appear suitable candidates for qubits encoding at low temperatures. We have exploited different functionalization pathways to graft molecular {Cr7Ni} rings onto a Au(111) surface from the liquid phase and we report a comparative analysis of the results obtained by STM, XPS, XAS and XMCD experimental techniques.
2010
- Probing local magnetization in molecular heterometallic Cr2Cu trimer
[Articolo su rivista]
Lorusso, Giulia; V., Corradini; A., Candini; A., Ghirri; Biagi, Roberto; Del Pennino, Umberto; S., Carretta; E., Garlatti; P., Santini; G., Amoretti; G., Timco; R. G., Pritchard; R. E. P., Winpenny; Affronte, Marco
abstract
We have extensively studied the magnetic features of a heterometallic molecular trimer, Cr2Cu. We havecharacterized molecular Cr2Cu by low-temperature ac susceptibility and specific-heat measurements and thisallowed us to determine the microscopic parameters of the spin Hamiltonian. Then, we used x-ray magneticcircular dichroism to selectively probe local symmetries, electronic configuration, orbital, and spin magneticmoments of the magnetic ions. We found that at low temperatures only the ground state S=3/2 is occupied andthe spin of Cu is found opposite to that of Cr in agreement with spin-Hamiltonian calculations.
2010
- Scanning tunnelling spectroscopy study of paramagnetic superconducting β′′-ET4[(H3O)Fe(C2O 4)3]C6H5Br crystals
[Articolo su rivista]
Gambardella, A.; Salluzzo, M.; Di Capua, R.; Affronte, M.; Gimenez-Saiz, C.; Gomez-Garca, C. J.; Coronado, E.; Vaglio, R.
abstract
Scanning tunnelling spectroscopy (STS) and microscopy (STM) were performed on the paramagnetic molecular superconductor β′′-ET 4[(H3O)Fe(C2O4)3]C 6H5Br. Under ambient pressure, this compound is located near the boundary separating superconducting and insulating phases of the phase diagram. In spite of a strongly reduced critical temperature Tc (Tc = 4.0K at the onset, zero resistance at Tc = 0.5K), the low temperature STS spectra taken in the superconducting regions show strong similarities with the higher Tc ET κ-derivatives series. We exploited different models for the density of states (DOS), with conventional and unconventional order parameters to take into account the role played by possible magnetic and non-magnetic disorder in the superconducting order parameter. The values of the superconducting order parameter obtained by the fitting procedure are close to the ones obtained on more metallic and higher Tc organic crystals and far above the BCS values, suggesting an intrinsic role of disorder in the superconductivity of organic superconductors and a further confirmation of the non-conventional superconductivity in such compounds. © 2010 IOP Publishing Ltd.
2010
- Surface-Enhanced Raman Signal for Terbium Single-Molecule Magnets Grafted on Graphene
[Articolo su rivista]
M., Lopes; A., Candini; M., Urdampilleta; A., Reserbat Plantey; V., Bellini; S., Klyatskaya; L., Marty; M., Ruben; AFFRONTE, Marco; W., Wernsdorfer; N., Bendiab
abstract
We report the preparation and characterization of monolayer graphene decorated withfunctionalized single-molecule magnets (SMMs). The grafting ligands provide a homogeneous and selectivedeposition on graphene. The grafting is characterized by combined Raman microspectroscopy, atomic forcemicroscopy (AFM), and electron transport measurements. We observe a surface-enhanced Raman signal thatallowed us to study the grafting down to the limit of a few isolated molecules. The weak interaction throughcharge transfer is in agreement with ab initio DFT calculations. Our results indicate that both molecules andgraphene are essentially intact and the interaction is driven by van der Waals forces.
2010
- Trioctahedral micas: relationships between crystal chemistry and magnetic behavior
[Abstract in Atti di Convegno]
Affronte, Marco; Brigatti, Maria Franca; Malferrari, Daniele; Marcelli, A.; Pini, S.
abstract
This study is aimed at verifying the factors affecting magneticsusceptibility χ variation in micas in relation to: I) Fe content;ii) Fe2+/Fe3+ ratio; iii) occurrence of Fe in tetrahedral andoctahedral sites, outlining possible ordering effects.Other aspects considered in our study will involve: i)eventually observed variation in magnetic properties forsamples showing similar chemical composition, but occurringfrom different areas and different crystallization environments;ii) possible influence of strong magnetic anomalies overmagnetic properties of samples therein occurring.All these aspects were thus addressed by characterizingsamples belonging to phlogopite – annite, tetra-ferriphlogopite– tetra-ferri-annite and polylithionite – siderophylliteseries. Some samples, representative of the whole sample set,were then selected for magnetic susceptibility characterization.Some of them also required calorimetric analyses, in order tobetter describe and understand their behaviour.Other experimental methods used in this investigationinclude: electron microprobe, structure resolution by X-raysingle crystal methods, X-ray absorption spectroscopy, with aparticular attention to the understanding of the XANES region.Main results demonstrate that: i) magnetic phenomenaassociated with maxima of magnetic susceptibility could beobserved at low temperature (T<10 K) and weak magnetic field(10 Oe); ii) magnetic properties depend not only on the total Fecontent. Samples where Fe is mostly in octahedral coordinationare predominantly ferromagnetic, strongly depending on the[6]Fe2+/([6]Fe2++[6]Fe3+) ratio, which was observed to increasewith Curie-Weiss θ constant (ranging from 13 to 24 K and from4.4 and 5.2 K for samples showing higher and lower[6]Fe2+/([6]Fe2++[6]Fe3+) ratio, respectively). In tetra-ferriphlogopite,dominant interactions were observed to be antiferromagnetic,as shown by a negative Curie-Weiss θ constantvalue (-25 K). Furthermore Curie-Weiss constant (θ) and thetemperature value giving the maximum in the magneticsusceptibility (Tm) seem to be dependent not only on the meannumber of Fe atoms in octahedral coordination (Z) per unit cell,but also on its ordering. Tm linearly depends on severalstructural parameters as well, e.g., the tetrahedral rotation angle α.
2009
- Engineering coupling between molecular spin qubits by coordination chemistry
[Articolo su rivista]
G. A., Timco; S., Carretta; F., Troiani; F., Tuna; R. J., Pritchard; E. J. L., Mcinnes; Ghirri, Alberto; A., Candini; P., Santini; G., Amoretti; Affronte, Marco; Richard E. P., Winpenny
abstract
Supramolecular chemistry enables nanoscale engineering of scalable structures,by introducing controlled interactions between well defined molecular building blocks. The ability to assemble weakly-interacting subsystems is a prerequisite for implementing quantum-information processing (QIP) and generating controlled entanglement . In recent years,molecular nanomagnets (MNMs) have been proposed as suitable candidates for the qubit encoding and manipulation . In particular,antiferromagnetic Cr7Ni rings at low temperature behave as effective spin-1/2 systems and exhibit long decoherence times . Here we show that these rings can be linked to each other through supramolecular functional groups,which allow an extensive tuning of the coupling between their spins. We demonstrate that maximally entangled states can be deterministically generated in tripartite supramolecular assemblies,formed by two Cr7Ni rings and a Cu ion,by simulating the system’s time evolution under the effect of realistic microwave pulse sequences.
2009
- Investigation of Li-doped MgB2
[Articolo su rivista]
Pallecchi, I.; Brotto, P.; Ferdeghini, C.; Putti, M.; Palenzona, A.; Manfrinetti, P.; Geddo Lehmann, A.; Orecchini, A.; Petrillo, C.; Sacchetti, F.; Affronte, M.; Allodi, G.; De Renzi, R.; Serventi, S.; Andreone, A.; Lamura, G.; Daghero, D.; Gonnelli, R. S.; Tortello, M.
abstract
We present a systematic characterization of Mg1-xLi xB2 polycrystalline samples with nominal Li content x up to 0.30. We explore the effectiveness of the substitution and investigate its influence on superconducting and normal state properties. The structural analyses by neutron and x-ray diffraction indicate that, despite the lattice parameters remaining unchanged within the experimental accuracy, around 30% of the nominal Li content actually enters the structure. Also the transition temperature is only weakly affected by Li substitution, but its relationship with the residual resistivity and magnetoresistivity data is compatible with a picture where π bands are filled with holes and/or affected by disorder, as predicted by theory. We also measured the magnetic penetration depth λ by an inductance technique. Data fits based on the standard Bardeen-Cooper- Schrieffer two-band model yield the zero-temperature limit of both λ and superconducting gaps: we find that λ(0) increases weakly and quickly saturates with increasing x, whereas Δπ(0) and Δσ(0) decrease with x. This analysis suggests that lithium substitution induces disorder mainly in the π band. Point contact spectroscopy results on samples obtained from the same batch are in full agreement with magnetic penetration depth data on the undoped sample, but give superconducting gaps almost insensitive to Li substitution, showing at most a small increase in Δπ(0) that may be related to an inhomogeneous distribution of Li content. © 2009 IOP Publishing Ltd.
2009
- Magnetic structure of the high-density single-valent eg Jahn-Teller system LaMn7 O12
[Articolo su rivista]
A., Prodi; E., Giglioli; R., Cabassi; F., Bolzoni; F., Licci; Q., Huang; A., Santoro; J. W., Lynn; Affronte, Marco; Marezio, A. G. a. u. z. z. i. M.
abstract
We studied the structural and physical properties of powder samples of the high-pressure LaMn7 O12 compound by means of neutron diffraction, dc magnetization, specific-heat, and dc electrical resistivity measurements. Our structural refinement shows that LaMn7 O12 is the counterpart with quadruple perovskite structure of the well-known simple-perovskite LaMnO3 because both compounds are single-valent Mn3+ systems sharing a similar pseudocubic network of buckled corner-sharing MnO6 octahedra distorted by the Jahn-Teller (JT) effect. Besides this similarity, LaMn7 O12 exhibits the following structural differences: (i) a monoclinic I2/m, instead of orthorhombic Pnma, structure; (ii) a much larger buckling of the MnO6 octahedra, corresponding to a Mn-O-Mn bond angle as small as ψ=180°-φ=136°. At TN,B =78K, the neutron data show evidence of an antiferromagnetic (AFM) structure of the octahedral Mn3+ ions (B sites), which consists of ferromagnetically coupled antiferromagnetic ac planes. This structure, commonly referred to as C type, markedly differs from the A -type structure of LaMnO3, which consists of antiferromagnetically coupled ferromagnetic ac planes. We argue that this difference may be due not only to the different buckling, because the magnetic superexchange interaction is sensitive to φ, but also to the existence of two distinct JT distorted B sites, a characteristic feature of the quadruple perovskite structure not found in LaMnO3. A further feature of this structure is the presence of a pseudocubic sublattice of JT Mn3+ ions with square coordination (A′ site) in addition to the B -site sublattice. At TN, A′ =21K, the A′ sublattice of LaMn7 O12 is found to form a second AFM structure consisting of AFM coupled ferromagnetic planes. This additional ordering appears to occur independently of that of the B sublattice. Finally, we observed a thermally activated insulating behavior of the resistivity similar to that previously reported for LaMnO3, which confirms the hypothesis that the superexchange interaction is predominant in single-valent systems while the double-exchange interaction is relevant only in mixed-valence or disordered systems. © 2009 The American Physical Society.
2009
- Magnetocaloric Effect in Spin-Degenerated Molecular Nanomagnets
[Articolo su rivista]
M., Evangelisti; A., Candini; Affronte, Marco; E., Pasca; L. J., de Jongh; R. T. W., Scott; E. K., Brechin
abstract
abstract
2009
- Molecular nanoMagnets for information technologies.
[Articolo su rivista]
Affronte, Marco
abstract
Molecular nanoMagnets for information technologies.
2009
- Octahedral cation ordering and magnetic behavior of micas
[Abstract in Rivista]
Pini, S.; Affronte, Marco; Brigatti, Maria Franca; Malferrari, Daniele; Marcelli, A.
abstract
This contribution is finalized at the discussion of the magnetic structure of two samples, belonging to phlogopite–annite [sample TK, chemical composition IV(Si2.76Al1.24) VI(Al0.64Mg0.72 Fe1.452+ Mn0.03Ti0.15) (K0.96Na0.05) O10.67 (OH)1.31 Cl0.02] and polylithionite–siderophyllite joints [sample PPB, chemical composition IV(Si3.14Al0.86)VI(Al0.75Mg0.01 Fe1.032+ Fe1.033+ Mn0.01Ti0.01Li1.09) (K0.99Na0.01) O10.00 (OH)0.65F1.35]. Samples differ for Fe ordering in octahedral sites, Fe2+/(Fe2+ + Fe3+) ratio, octahedral composition, defining a different environment around Fe cations, and layer symmetry. Spin-glass behavior was detected for both samples, as evidenced by the dependency of the temperature giving the peak in the susceptibility curve from the frequency of the applied alternating current magnetic field. The crystal chemical features are associated to the different temperature at which the maximum in magnetic susceptibility is observed: 6 K in TK, where Fe is disordered in all octahedral sites, and 8 K in PPB sample, showing a smaller and more regular coordination polyhedron for Fe, which is ordered in the trans-site and in one of the two cis-sites.
2009
- Probing edge magnetization in antiferromagnetic spin segments
[Articolo su rivista]
A., Ghirri; G., Lorusso; F., Moro; V., Corradini; Affronte, Marco; J. C., Cezar; C., Muryn; F., Tuna; G., Timco; R. E. P., Winpenny
abstract
abstract
2009
- Specific heat investigation in high magnetic field of the magnetic ordering of the rare-earth lattice in RFeAsO: The case of Sm
[Articolo su rivista]
S., Riggs; C., Tarantini; J., Jaroszynski; A. Gurevich A., Palenzona; M., Putti; T., Duc Nguyen; Affronte, Marco
abstract
abstract
2009
- Successful grafting of isolated molecular Cr7Ni rings on Au(111) surface
[Articolo su rivista]
V., Corradini; Moro, Fabrizio; Biagi, Roberto; DE RENZI, Valentina; DEL PENNINO, Umberto; V., Bellini; S., Carretta; P., Santini; V. A., Milway; G., Timco; R. E. P., Winpenny; Affronte, Marco
abstract
We deeply investigated the properties of submonolayer distributions of isolated molecular Cr7Ni ringsdeposited on Au111 by liquid phase. X-ray absorption spectra measured at the Cr and Ni L2,3 edges show thatthe grafting of the Cr7Ni rings onto the gold surface does not affect the oxidation state and the local symmetryof the Cr and Ni sites. The circular dichroism shows a change in sign of the Ni magnetic moment. This is dueto a reduction in the exchange coupling constants that, however, preserves the structure of the low-energylevels of the grafted rings, as corroborated by spin-Hamiltonian simulations and comparison with measure-ments on bulk sample. Density-functional theory calculations show that the Ni-Cr bond gets weaker with slightring distortion suggesting possible explanation for the observed magnetic behavior. These results show thatcomplex magnetic molecules can be grafted onto surfaces, and that changes in their magnetic behavior must beexamined in individual cases.
2008
- Ab initio study on a chain model of the Cr8 molecular magnet
[Articolo su rivista]
D. M., Tomecka; V., Bellini; F., Troiani; Manghi, Franca; G., Kamieniarz; Affronte, Marco
abstract
We present a density-functional theory investigation of the electronic and magnetic properties of a linear chain model of the antiferromagnetic Cr8 molecular ring. The chain model system is characterized by a smaller size of the simulation cell needed to perform the calculations, with respect to the one necessary for Cr8. By the thorough comparison between the model complex and the Cr8 ring, we prove that the chain model is reliable and mimics with good approximation the electronic and magnetic properties of Cr8.
2008
- Comparison among superconducting models for β’’-ET4[(H3O)Fe(C2O4)3]·C6H5Br single crystals by scanning tunnelling spectroscopy
[Articolo su rivista]
A., Gambardella; R., Di Capua; M., Salluzzo; R., Vaglio; Affronte, Marco; DEL PENNINO, Umberto; S., Curreli; C., Giménez Saiz; C. J., Gómez García; E., Coronado
abstract
Single crystals of the novel superconductor β’’-(BEDT-TTF)4[(H3O)Fe(C2O4)3]·C6H5Br charge-transfer salt were studied using a Scanning Tunnelling Microscope. The measured samples have an onset critical temperature of about 4.0 K. Features often reported on similar compounds were observed in the tunnelling spectra at 1.4 K. STS spectra are compared with several models for the superconducting density of states. Our analysis evidences inhomogeneous superconductivity and indicates that the presence of a magnetic layer into the sample plays a role in determining the superconducting spectroscopic features.
2008
- Decoherence induced by hyperfine interactions with nuclearspins in antiferromagnetic molecular rings
[Articolo su rivista]
F., Troiani; V., Bellini; Affronte, Marco
abstract
Decoherence induced by hyperfine interactions with nuclearspins in antiferromagnetic molecular rings
2008
- From single-molecule magnetism to long-range ferromagnetism in Hpyr [Fe17 O16 (OH) 12 (py) 12 Br4] Br4
[Articolo su rivista]
Vecchini, C.; Ryan, D. H.; Cranswick, L. M. D.; Evangelisti, M.; Kockelmann, W.; Radaelli, P. G.; Candini, A.; Affronte, M.; Gass, I. A.; Brechin, E. K.; Moze, O.
abstract
The molecular magnet Hpyr [Fe17 O16 (OH) 12 (py) 12 Br4] Br4 (" Fe17 ") has a well-defined cluster spin ground state of S=35/2 at low temperatures and an axial molecular anisotropy of only D-0.02K. Dipolar interactions between the molecular spins induce long-range magnetic order below 1.1 K. We report here the magnetic structure of Fe17, as determined by unpolarized neutron diffraction experiments performed on a polycrystalline sample of deuterated Fe17 in zero applied magnetic field. In addition, we report bulk susceptibility, magnetization, and specific heat data. The temperature dependence of the long-range magnetic order has been tracked and is well accounted for within mean-field theory. Ferromagnetic order along the crystallographic c axis of the molecular spins, as determined by the neutron diffraction experiments, is in agreement with ground-state dipolar energy calculations. © 2008 The American Physical Society.
2008
- Long-range ferromagnetic order in the giant-spin molecular nanomagnet Fe17 (Hpyr[Fe17O16(OH)12(py)12Br4]Br4)
[Articolo su rivista]
C., Vecchini; D. H., Ryan; L. M. D., Cranswick; M., Evangelisti; W., Kockelmann; A., Candini; Affronte, Marco; I. A., Gass; E. K., Brechin; O., Moze
abstract
magnetic order in molecular magnets
2008
- Magnetic behaviour of trioctahedral mica-2M1 occurring in amagnetic anomaly zone
[Articolo su rivista]
Pini, Stefano; Affronte, Marco; Brigatti, Maria Franca
abstract
This work relates the crystal chemistry and the magnetic behaviour of a trioctahedral mica (chemical formula: K0.90Na0.01Ca0.01Ba0.01&0.07)(Al0.05Fe2+ 1.20Mg1.38Ti0.32Mn0.01&0.04)(Al1.12Si2.88)O10 (F0.27OH1.27O0.46); unit cell parameters: a = 5.345(2), b = 9.261(4), c = 20.189(8) A; b =95.075(8)º) from Minto Block (Ungava peninsula, northern Quebec, Canada), a region characterized by high magnetic anomalies. Crystallographic and X-ray absorption spectroscopy data suggest a prevalentdivalent oxidation state for Fe and a disordered Fe2+ distribution in the two octahedralsites M1 and M2. The realpart of magnetic susceptibility shows two peaks at ~5.2 K and 120 K. However, asdemonstrated by AC magnetic susceptibility measurements, the origin of the two effects is different: the peak position of the first one (i.e. the effect revealed at 5.2 K) is frequency-dependent, thus suggesting a spin-glass like behaviour. The effect at 120 K can instead be attributed to the occurrence of diluted phases in mica matrix, such as Fe oxides.
2008
- Magnetic imaging of prussian blue nanoparticles grafted on FIB-patterned substrates
[Articolo su rivista]
A., Ghirri; A., Candini; M., Evangelisti; G. C., Gazzadi; Affronte, Marco; F., Volatron; D., Brinzei; B., Fleury; L., Catala; C., David; T., Mallah
abstract
Magnetic imaging of prussian blue nanoparticles grafted on FIB-patterned substrates
2008
- Slow magnetic dynamics in the Ni10 family of compounds
[Articolo su rivista]
S., Carretta; P., Santini; G., Amoretti; Affronte, Marco; Candini, Andrea; Ghirri, Alberto; I. S., Tidmarsh; R. H., Laye; R., Shaw; E. J. L., Mcinnes
abstract
slow relaxation of magnetization
2008
- Supertetrahedral and bi-supertetrahedral cages: Synthesis, structures, and magnetic properties of deca- and enneadecametallic cobalt(II) clusters
[Articolo su rivista]
L., Lisnard; F., Tuna; A., Candini; Affronte, Marco; R. E. P., Winpennya; Eric J. L., Mcinnes
abstract
Supertetrahedral and bi-supertetrahedral cages: synthesis, structures and magnetic properties of deca- and enneadeca-metallic cobalt(II) clusters.
2008
- Thermal properties of SmFeAsO1-x Fx as a probe of the interplay between electrons and phonons
[Articolo su rivista]
Tropeano, M.; Martinelli, A.; Palenzona, A.; Bellingeri, E.; Galleani D'Agliano, E.; Nguyen, T. D.; Affronte, M.; Putti, M.
abstract
A comparative study of thermal properties of SmFeAsO, SmFeAs (O 0.93 F0.07), and SmFeAs(O0.85 F0.15) samples is presented. Specific heat and thermal conductivity show clear evidence of the spin-density wave (SDW) ordering below TSDW ∼ 135 K in undoped SmFeAsO. At low level of F doping, SmFeAs (O0.93 F 0.07), SDW ordering is suppressed and superconducting features are not yet optimally developed in both specific heat and thermal conductivity. At optimal level of F-doping SmFeAs (O0.85 F0.15) anomalies related to the superconducting transition are well noticeable. By a compared analysis of doped and undoped samples we conclude that despite the fact that F doping modifies definitely the electronic ground state, it does not substantially alter phonon and electron parameters, such as phonon modes, Sommerfeld coefficient, and electro-phonon coupling. The analysis of the thermal conductivity curves provides an evaluation of the SDW and superconducting energy gap, showing that phonons can suitably probe features of the electronic ground state. © 2008 The American Physical Society.
2008
- Thermal properties of SmFeAs(O1-xFx) as probe of the interplay between electrons and phonons.
[Articolo su rivista]
M., Tropeano; A., Martinelli; A., Palenzona; E., Bellingeri; E., Galleani d’Agliano; T. D., Nguyen; Affronte, Marco; M., Putti
abstract
Thermal properties of SmFeAs(O1-xFx) as probe of the interplay between electrons and phonons.
2008
- Two-Step Magnetic Ordering in Quasi-One-Dimensional Helimagnets: Possible Experimental Validation of Villain’s Conjecture about a Chiral Spin Liquid Phase
[Articolo su rivista]
F., Cinti; A., Rettori; M. G., Pini; M., Mariani; E., Micotti; A., Lascialfari; N., Papinutto; A., Amato; A., Caneschi; D., Gatteschi; Affronte, Marco
abstract
Low-temperature specific heat, magnetic susceptibility, and zero-field muon spin resonance (SR)measurements have been performed in the quasi-one-dimensional molecular helimagnetic compoundGdhfac3NITEt. The specific heat presents two anomalies at T02:190:02K and TN 1:880:02K,which both disappear upon the application of a weak magnetic field. Conversely, magnetic susceptibilityand SR data show the divergence of two-spin correlation functions only at TN 1:88 0:02 K. Theseresults suggest an experimental validation of Villain’s conjecture of a two-step magnetic ordering in quasione-dimensional XY helimagnets; i.e., the paramagnetic phase and the helical spin solid phase areseparated by a chiral spin liquid phase, where translational invariance is broken without violation ofrotational invariance.
2008
- X-ray magnetic circular dichroism investigation of spin and orbital moments in Cr8 and Cr7Ni antiferromagnetic rings
[Articolo su rivista]
V., Corradini; Moro, Fabrizio; Biagi, Roberto; DEL PENNINO, Umberto; DE RENZI, Valentina; S., Carretta; P., Santini; Affronte, Marco; J. C., Cezar; G. TIMCO AND R. E. P., Winpenny
abstract
We investigated the electronic and magnetic properties of thick films of molecular Cr8 and Cr7Ni antiferromagneticrings by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroismXMCD. We determined the local symmetries, the electronic configuration, and the values of orbital and spinmoments at the Cr and Ni sites of the molecular rings. XMCD measurements show that the correlation betweenthe Cr and Ni spins in the Cr7Ni molecular ring switches from antiferromagnetic to ferromagnetic withincreasing temperature. Experimental data are interpreted using XMCD sum rules that allow the separateevaluation of the spin and the orbital contributions to the total magnetic moment of the ring as a function oftemperature and magnetic field. The magnetic behaviors experimentally observed are compared with the resultsof spin-Hamiltonian calculations, based on microscopic parameters derived by inelastic-neutron scattering andlow-temperature specific-heat measurements. The very good agreement between experimental data and calculationsis a clear indication of the integrity of molecules. The temperature dependence of the ion magneticmoments results from the interplay between Zeeman and isotropic-exchange contributions, and is well capturedby the theoretical model.
2007
- A ferromagnetic mixed-valent Mn supertetrahedron: towards low-temperature magnetic refrigeration
[Articolo su rivista]
M., Manoli; R. D. L., Johnstone; S., Parsons; M., Murrie; Affronte, Marco; M. EVANGELISTI AND E. K., Brechin
abstract
Herein we report thesynthesis and structure of a decametallic mixed-valent Mnsupertetrahedron with the ligand 2-amino-2-methyl-1,3-propanediol(ampH2); this cluster displays dominant ferromagneticexchange and a spin ground state of S=22. Furthermore,we show that the magnetic behavior of the compound isparticularly appealing in view of its possible application as anefficient low-temperature magnetic refrigerant.
2007
- A Ring Cycle: Studies of Heterometallic Wheels
[Articolo su rivista]
Affronte, Marco; S., Carretta; G. A., Timco; R. E. P., Winpenny
abstract
The synthesis of a series of heterometallic rings and chains is reported. The family is based on theoctanuclear cages of general formula [H2NR2][M7M9F8(O2CR9)16], where M is a trivalent metal(Cr, Fe, V, Al, Ga or In), M9 is a divalent metal (Mn, Fe, Co, Ni, Mg, Zn, Cd), R is a linear alkylchain and O2CR9 is one of around twenty carboxylates. Other members of the family withnonametallic and decametallic cores are described, and some new physics is outlined, includinginitial investigations of the proposed application of {Cr7Ni} rings as Qubits in quantuminformation processing.
2007
- Elementary excitations in antiferromagnetic Heisenberg spin segments.
[Articolo su rivista]
A., Ghirri; A., Candini; Affronte, Marco; M., Evangelisti; S., Carretta; P. SANTINI AND G., Amoretti; R. S. G., Davies; G., TIMCO AND; R. E. P., Winpenny
abstract
We report on ac susceptibility, low-temperature magnetization, and specific heat measurements on molecularcompounds, shortly named Cr62, Cr72, Cr8Cd, and NiCr62Zn, that comprise different variants of spinarrays. These systems constitute real examples of collections of identical antiferromagnetic Heisenberg spinsegments. We indeed show that this picture, with dominant exchange term in the spin Hamiltonian J/kBranging from 13 to 16 K in all compounds and weak anisotropy term, fits well the measured physical properties.The character of energy spectra and the low-lying magnetic excitations are discussed accordingly. Thedirect comparison of experimental results and of the energy spectra of these variants with those of similarcyclic spin systems evidences effects associated with i the breaking the cyclic boundary conditions and iiodd and even nuclearities of spin segments.
2007
- Evidence for a helical and a chiral phase transition in the Gd (hfac)3 NITiPr magnetic specific heat
[Articolo su rivista]
Cinti, F.; Rettori, A.; Barucci, M.; Olivieri, E.; Risegari, L.; Ventura, G.; Caneschi, A.; Gatteschi, D.; Rovai, D.; Pini, M. G.; Affronte, M.; Mariani, M.; Lascialfari, A.
abstract
New specific heat data taken at very low temperatures (0.03 < T < 0.18 K) in quasi one-dimensional helimagnet Gd (hfac)3 NITiPr show a clear λ anomaly at TN = 0.039 K signaling the onset of the 3D helimagnetic phase. They match fairly well with previously reported data which showed the onset of the chiral phase transition at T0 = 2.08 K. Also new magnetic susceptibility data taken in the neighborhood at T0 are repeated. © 2006 Elsevier B.V. All rights reserved.
2007
- Focused ion beam patterned Hall nano-sensors
[Articolo su rivista]
Candini, A.; Gazzadi, G. C.; di Bona, A.; Affronte, M.; Ercolani, D.; Biasiol, G.; Sorba, L.
abstract
By means of focused ion beam milling, we fabricate Hall magnetometers with active areas as small as 100 × 100 nm2. The constituent material can either be metallic (Au), semimetallic (Bi) or doped bulk semiconducting (Si doped GaAs). We experimentally show that Au nano-probes can work from room temperature down to liquid helium with magnetic flux sensitivity < 10- 1 Φ0. © 2006 Elsevier B.V. All rights reserved.
2007
- Isolated heterometallic Cr7Ni rings grafted on Au(111) surface
[Articolo su rivista]
V., Corradini; Biagi, Roberto; DEL PENNINO, Umberto; DE RENZI, Valentina; Gambardella, Alessandro; Affronte, Marco; Ca, Muryn; Ga, Timco; Rep, Winpenny
abstract
A study of the deposition of heterometallic antiferromagnetically coupled rings onto gold surfaces is reported. Twonew {Cr7Ni} rings, [NH2nPr2][Cr7NiF8(3-tpc)16] (1) (where 3-tpc ) 3-thiophenecarboxylate) and [nBuNH2CH2CH2SH][Cr7NiF8(O2CtBu)16] (2) have been made and structurally characterized. They have been deposited from the liquidphase on Au(111) and the adsorbed molecules compared by means of scanning tunneling microscopy (STM) andX-ray photoemission spectroscopy (XPS). In both cases a two-dimensional distribution of individually accessible{Cr7Ni} heterometallic rings on the gold surface has been obtained, exploiting the direct grafting of sulfur-functionalizedclusters. There is a competition between the chemisorption of the {Cr7Ni} clusters and a thiolic self-assembledmonolayer (SAM) formed by free ligands. In 2, the presence of a single sulfur ligand should force the molecule tograft with the ring axis normal to the surface. The cluster stability in the STM images and the S-2p energy positionsdemonstrate, for both functionalizations, the strength of the grafting with the gold surface.
2007
- Molecular spin clusters for quantum computation
[Articolo su rivista]
Affronte, M.; Troiani, F.; Ghirri, A.; Carretta, S.; Santini, P.; Schuecker, R.; Timco, G.; Winpenny, R. E. P.
abstract
We present our recent progresses on the implementation of qubits with molecular-spin clusters. Here, we focus on molecular linkers, made of Ru dimers with different electronic states and designed to couple two Cr7Ni molecular rings with spin S = 1 / 2 as ground state. © 2006 Elsevier B.V. All rights reserved.
2007
- Neutron irradiation effects on two gaps in MgB2
[Articolo su rivista]
Ferrando, V.; Affronte, M.; Daghero, D.; Di Capua, R.; Tarantini, C.; Putti, M.
abstract
In this paper, an overview of the gaps behaviour as a function of disorder in neutron-irradiated samples is presented. The general effects of neutron irradiation is discussed and, by comparing the different experiments performed in literature, we show that this technique is very effective to introduce disorder in a controlled way in MgB2. Then, the three experiments that allowed determining the two gaps, i.e. specific heat, point contact spectroscopy and scanning tunnelling spectroscopy measurements are analysed and compared, and the role of interband scattering is discussed. © 2007.
2007
- Single molecule magnets for quantum computation
[Articolo su rivista]
Affronte, Marco; Troiani, Filippo; Ghirri, Alberto; Candini, Andrea; M., Evangelisti; Corradini, Valdis; S., Carretta; P., Santini; G., Amoretti; F., Tuna; G., Timco; R. E. P., Winpenny
abstract
We present recent achievements and perspectives for the encoding of qubitswith molecular spin clusters.
2007
- Spin triangles as optimal units for molecule-based quantum gates
[Articolo su rivista]
S., Carretta; P., Santini; G., Amoretti; Troiani, Filippo; Affronte, Marco
abstract
We show that isosceles antiferromagnetic spin triangles constitute simple and very advantageous units forimplementing quantum gates with magnetic molecules. Indeed, the spin structure of their low-energy wavefunctions enables switchable effective interqubit couplings even in the presence of permanent microscopicinteractions. The great advantage of the proposed hardware is that no fine tuning of microscopic intermolecularinteractions is required. This significantly increases the robustness with respect to disorder of both the local andglobal manipulation approaches.
2007
- Vacancy-driven magnetocaloric effect in Prussian blue analogues
[Articolo su rivista]
Evangelisti, M.; Manuel, E.; Affronte, M.; Okubo, M.; Train, C.; Verdaguer, M.
abstract
We experimentally show that the magnetocaloric properties of molecule-based Prussian blue analogues can be adjusted by controlling during the synthesis the amount of intrinsic vacancies. For Csx Ni4II [CrIII (CN)6](8 + x) / 3, we find indeed that the ferromagnetic phase transition induces significantly large magnetic entropy changes, whose maxima shift from ∼ 68 to ∼ 95 K by varying the number of [CrIII (CN)6]3 - vacancies, offering a unique tunability of the magnetocaloric effect in this complex. © 2007 Elsevier B.V. All rights reserved.
2007
- Valence tautomerism interconversion triggers transition to stable charge distribution in solid polymeric cobalt-polyoxolene complexes
[Articolo su rivista]
Affronte, Marco; A., Beni; A., Dei; L., Sorace
abstract
The reaction of 3,3,3,3-tetramethyl-5,5,6,6-tetrahydroxy-1,1spiro-bis(indane), L, with cobalt salts inthe presence of diazine ligands affords polymeric derivatives of general formula[CoL(diazine)]∞·nH2O, L being coordinated as a mixed-valence SQ-Cat species. The bipiridyl ando-phenantroline derivatives have been characterised by means of magnetic, EPR and calorimetricmeasurements. The characterisation showed that both systems are obtained in a metastable chargedistribution, presumably trapped due to the class I character of the mixed valence form of the ligand.The occurrence of entropy-driven valence tautomeric interconversion induces the transition to the moststable charge distribution. The photomagnetic characterisation evidenced that valence tautomericprocess can be photoinduced and that the lifetime of the photoinduced metastable state, obtained inlow yield, is of ca. 2 × 105 s at 9 K.
2007
- 1,2,3-Triazole-bridged tetradecametallic transition metal clusters [M14(L)6O6(OMe)18X6] (M = FeIII, CrIII and VIII/IV), and related compounds: ground state spins ranging from S = 0 to 25 and spin-enhanced magnetocaloric effect.
[Articolo su rivista]
Rachel S., Shaw; Rebecca H., Laye; Leigh F., Jones; David M., Low; Caytie Talbot, Eeckelaers; Qiang, Wei; Constantinos J., Milios; Simon, Teat; Madeliene, Helliwell; James, Raftery; Marco, Evangelisti; Affronte, Marco; David, Collison; Euan K., Brechin; Eric J. L., Mcinnes
abstract
We report the synthesis, by solvothermal methods, of the tetradecametallic cluster complexes [M14(L)6O6(OMe)18-Cl6] (M ) FeIII, CrIII) and [V14(L)6O6(OMe)18Cl6-xOx] (L ) anion of 1,2,3-triazole or derivative). Crystal structuredata are reported for the {M14} complexes [Fe14(C2H2N3)6O6(OMe)18Cl6], [Cr14(bta)6O6(OMe)18Cl6] (btaH )benzotriazole), [V14O6(Me2bta)6(OMe)18Cl6-xOx] [Me2btaH ) 5,6-Me2-benzotriazole; eight metal sites are VIII, theremainder are disordered between {VIII-Cl}2+ and {VIVdO}2+] and for the distorted [FeIII14O9(OH)(OMe)8(bta)7-(MeOH)5(H2O)Cl8] structure that results from non-solvothermal synthetic methods, highlighting the importance oftemperature regime in cluster synthesis. Magnetic studies reveal the {Fe14} complexes to have ground state electronicspins of S e 25, among the highest known, while in contrast the {Cr14} complex has an S ) 0 ground statedespite having a very similar structure and all complexes being dominated by intramolecular antiferromagneticexchange interactions. The {Fe14} complexes undergo a magnetic phase transition to long-range ordering at relativelyhigh temperatures for molecular species, which are governed by the steric bulk of the triazole (TN ) 1.8 and 3.4K for L ) bta- and H2C2N3-, respectively). The huge spins of the {Fe14} complexes lead to very large magnetocaloriceffects (MCE)sthe largest known for any material below 10 Kswhich is further enhanced by spin frustration withinthe molecules due to the competing antiferromagnetic interactions. The largest MCE is found for [Fe14-(C2H2N3)6O6(OMe)18Cl6] with an isothermal magnetic entropy change -¢Sm of 20.3 J kg-1 K-1 at 6 K for anapplied magnetic field change of 0-7 T.
2006
- Codoped Mg1-x(AlLi)xB2 compounds: a key system to probe the effect of charge doping and inter-band scattering
[Articolo su rivista]
M. Monni C., Ferdeghini; M., Putti; P. Manfrinetti A., Palenzona; Affronte, Marco; P., Postorino; M., Lavagnini; A., Sacchetti; D., Di Castro; F., Sacchetti; C., Petrillo; A., Orecchini
abstract
We prepared a series of Mg1−xAlLixB2 samples with 0x0.45 in order to compensate with Li theelectron doping induced by Al. Structural characterization by means of neutron and x-ray diffraction confirmsthat Li enters the MgB2 structure even though in an amount less than nominal one. We performed susceptibility,resistivity, and specific heat measurements. Vibrational properties were also investigated by means ofRaman spectroscopy. We compare these results with those obtained on a homologous series of Mg1−xAlxB2samples. The systematic success of scaling the relevant properties with the Al content rather than with theelectron doping suggests that lattice deformation plays an important role in tuning the superconductingproperties.
2006
- Hall nano-probes fabricated by focused ion beam
[Articolo su rivista]
A., Candini; Cg, Gazzadi; A., Di Bona; Affronte, Marco; D., Ercolani; G., Biasiol; Sorba, Lucia
abstract
Hall magnetometers with active areas down to 100 x 100 nm(2) were fabricated patterning gold and Si-doped GaAs films by focused ion beam. For GaAs probes, electrical characterization shows that the magnetic flux sensitivity is better than 10(-2)Phi(0) at room temperature. Hall nano-probes made of gold can work down to liquid helium temperature with magnetic flux sensitivity 10(-1)Phi(0).
2006
- High-temperature slow relaxation of the magnetization in Ni-10 magnetic molecules
[Articolo su rivista]
Carretta, S; Santini, P; Amoretti, G; Affronte, Marco; Candini, A; Ghirri, A; Tidmarsh, Is; Laye, Rh; Shaw, R; Mcinnes, Ejl
abstract
We investigate a family of molecular crystals containing noninteracting Ni-10 magnetic molecules. We find slow relaxation of the magnetization below a temperature as high as 17 K and we show that this behavior is not associated with an anisotropy energy barrier. Ni-10 has a characteristic magnetic energy spectrum structured in dense bands, the lowest of which makes the crystal opaque to phonons of energy below about 1 meV. We ascribe the nonequilibrium behavior to the resulting resonant trapping of these low-energy phonons. Trapping breaks up spin relaxation paths leading to a novel kind of slow magnetic dynamics which occurs in the lack of anisotropy, magnetic interactions and quenched disorder.
2006
- Magnetocaloric effect in hexacyanochromate Prussian blue analogs
[Articolo su rivista]
Esprana, Manuel; Marco, Evangelisti; Affronte, Marco; Masashi, Okubo; Cyrille, Train; AND MICHEL, Verdaguer
abstract
magnetocaloric effect in prussian blue compounds
2006
- Magnetothermal properties of molecule-based materials
[Articolo su rivista]
Evangelisti, M; Luis, F; de Jongh, Lj; Affronte, Marco
abstract
We critically review recent results obtained by studying the low-temperature specific heat of some of the most popular molecule-based materials. After introducing the experimental techniques and basic theoretical framework needed for heat capacity determination and understanding, we report on the magnetothermal properties of molecular antiferromagnetic wheels. For selected molecular high-spin clusters, particular emphasis is devoted to magnetic quantum tunnelling and coherence as well as collective phenomena as probed by heat capacity experiments. We discuss also the possibilities of application of molecule-based materials for magneto-cooling at low temperatures and the limitations in other temperature ranges. Perspectives for future developments are mentioned as well.
2006
- Molecular routes for spin cluster qubits
[Articolo su rivista]
AFFRONTE, Marco; Troiani, F; Ghirri, A; Carretta, S; Santini, P; Corradini, V; Schuecker, R; Muryn, C; Timco, G; Winpenny, RE
abstract
We report on real molecular complexes and propose strategies that explore the possibility of implementation of specific quantum computation architectures with molecular spin systems. We focus on Cr3+ carboxylate derivatives and use the Loss - DiVincenzo scheme as reference.
2006
- Observation of the Crossover from Two-Gap to Single-Gap Superconductivity through Specific Heat Measurements in Neutron Irradiated MgB2
[Articolo su rivista]
M., Putti; Affronte, Marco; C., Ferdeghini; P., Manfrinetti; C., Tarantini; E., Lehmann
abstract
We report specific heat measurements on neutron-irradiated MgB2 samples, for which the criticaltemperature is lowered to 8.7 K, but the superconducting transition remains extremely sharp, indicative ofa defect structure extremely homogeneous. Our results evidence the presence of two superconducting gapsin the temperature range above 21 K, while single-gap superconductivity is well established as a bulkproperty, not associated with local disorder fluctuations, when Tc decreases to 11 K.
2006
- Role of charge doping and lattice distortions in codoped Mg1-x (AlLi)x B2 compounds
[Articolo su rivista]
Monni, M.; Ferdeghini, C.; Putti, M.; Manfrinetti, P.; Palenzona, A.; Affronte, M.; Postorino, P.; Lavagnini, M.; Sacchetti, A.; Di Castro, D.; Sacchetti, F.; Petrillo, C.; Orecchini, A.
abstract
We prepared a series of Mg1-x (AlLi)x B2 samples with 0≤x≤0.45 in order to compensate with Li the electron doping induced by Al. Structural characterization by means of neutron and x-ray diffraction confirms that Li enters the Mg B2 structure even though in an amount less than nominal one. We performed susceptibility, resistivity, and specific heat measurements. Vibrational properties were also investigated by means of Raman spectroscopy. We compare these results with those obtained on a homologous series of Mg1-x Alx B2 samples. The systematic success of scaling the relevant properties with the Al content rather than with the electron doping suggests that lattice deformation plays an important role in tuning the superconducting properties. © 2006 The American Physical Society.
2006
- Tunable dipolar magnetism in high-spin molecular clusters
[Articolo su rivista]
Evangelisti, M; Candini, A; Ghirri, A; Affronte, Marco; Powell, Gw; Gass, Ia; Wood, Pa; Parsons, S; Brechin, Ek; Collison, D; Heath, Sl
abstract
We report on the Fe-17 high-spin molecular cluster and show that this system is an exemplification of nanostructured dipolar magnetism. Each Fe-17 molecule, with spin S=35/2 and axial anisotropy as small as D similar or equal to-0.02 K, is the magnetic unit that can be chemically arranged in different packing crystals while preserving both the spin ground state and anisotropy. For every configuration, molecular spins are correlated only by dipolar interactions. The ensuing interplay between dipolar energy and anisotropy gives rise to macroscopic behaviors ranging from superparamagnetism to long-range magnetic order at temperatures below 1 K.
2005
- AntiFerromagnetic molecular rings for quantum computation.
[Relazione in Atti di Convegno]
Affronte, Marco; Troiani, Filippo; Ghirri, Alberto; S., Carretta; P., Santini; G., Amoretti; S., Piligkos; G., Timco; R. E. P., Winpenny
abstract
Molecular magnets have been recently proposed as possible building blocks for a solid-state quantum computer. In order to substantiateand develop such a proposal, one needs to identify those molecules that are best suited for the qubit encoding and manipulation.Here, we focus on a heterometallic molecular ring, namely Cr7Ni, where the substitution of one Cr3+(S = 3/2) withNi2+(S = 1) provides an extra spin to the otherwise compensated molecule. We show that its ground state consists in an S = 1/2doublet, energetically well separated (D0/kB 13 K at zero magnetic field) from the first excited multiplet. This relatively large valueof D0, together with the reduced mixing of the subspaces corresponding to different values of the total spin S, enables a safe encodingof the |0æ and |1æ states with the ground-state doublet, and allows to coherently rotate the effective S = 1/2 spin, while keeping thepopulation loss to the excited states negligible. A further, intriguing challenge is represented by the implementation of the conditionaldynamics (two-qubit gates). We present here preliminary characterization of molecular ‘‘Cr7Ni-dimers’’, i.e., derivatives inwhich two Cr7Ni rings are linked with each other by means of delocalized aromatic amines. The resulting intercluster couplingsare estimated to be 61 K and are expected to be permanent, i.e., not tuneable during gating, as required by the standard approachto quantum computation. We discuss a computational scheme that allows in principle to overcome this limitation. The most relevantdecoherence mechanisms for Cr7Ni and possible ways to reduce their effects are discussed as well.
2005
- Chiral and helical phase transitions in quasi-1D molecular magnets
[Articolo su rivista]
Cinti, F; Affronte, Marco; Lascialfari, A; Barucci, M; Olivieri, E; Pasca, E; Rettori, A; Risegari, L; Ventura, G; Pini, Mg; Cuccoli, A; Roscilde, T; Caneschi, A; Gatteschi, D; Rovai, D.
abstract
We review recent experimental data obtained with quasi-one-dimensional (1D) molecular magnets Gd(hfaC)(3)NITR. For Gd(hfac)(3)NITiPr the results can be coherently explained only in terms of the Villain´s conjecture: a chiral order at intermediate temperatures and helical order at low temperatures. In the case of weakly frustrated molecular magnetic chain compounds Gd(hfac)(3)-NITPh, new specific heat measurements show the presence of a phase transition to the three-dimensional (3D) helical order, observed at T similar to 0.63 K. In the temperature range 25-250 mK the experimental specific heat data are reproduced very well by a simple spin wave theory.
2005
- Lattice effects in the ferromagnetic insulating phase of manganites
[Articolo su rivista]
M., Plate; F., Bondino; M., Zacchigna; M., Zangrando; I., Alessandri; Affronte, Marco; L., Malavasi; F., Parmigiani
abstract
Lattice effects in the ferromagnetic insulating phase of manganites
2005
- Linking Rings Through Diamines and Clusters: Exploring Synthetic Methods for Making Magnetic Quantum Gates
[Articolo su rivista]
AFFRONTE, Marco; I., CASSON; M., EVANGELISTI; A., CANDINI; S., CARRETTA; C. A., MURYN; S. J., TEAT; G. A., TIMCO; W., WERNSDORFER; AND R. E. P., WINPENNY
abstract
Linking Rings Through Diamines and Clusters:
2005
- Molecular Engineering of antiferromagnetic rings for quantum computation.
[Articolo su rivista]
F., Troiani; Ghirri, Alberto; Affronte, Marco; S., Carretta; P., Santini; G., Amoretti; S., Piligkos; G., Timco; R. E. P., Winpenny
abstract
The substitution of one metal ion in a Cr-based molecular ring with dominant antiferromagneticcouplings allows the engineering of its level structure and ground-state degeneracy. Here we characterize aCr7Ni molecular ring by means of low-temperature specific-heat and torque-magnetometry measurements,thus determining the microscopic parameters of the corresponding spin Hamiltonian. The energyspectrum and the suppression of the leakage-inducing S mixing render the Cr7Ni molecule a suitablecandidate for the qubit implementation, as further substantiated by our quantum-gate simulations.
2005
- Molecular nanoclusters as magnetic refrigerants: The case of Fe-14 with very large spin ground-state
[Articolo su rivista]
Evangelisti, M; Candini, A; Ghirri, A; Affronte, Marco; Piligkos, S; Brechin, Ek; Mcinnes, Ejl
abstract
We report on the magnetic and thermal properties of the Fe-14 Molecular nanocluster. We find a huge magnetocaloric response in the temperature range below 10 K. This is to large extent caused by its very large spin ground-state combined with an excess of entropy arising from the admixture of low-lying excited S states. We also show that the high degree of symmetry of the Fe-14 cluster core, resulting in a very small cluster magnetic anisotropy, enables the Occurrence of long-range antiferromagnetic order below T-N = 1.87 K.
2005
- Proposal for quantum gates in permanently coupled antiferromagnetic spin rings without need of local fields
[Articolo su rivista]
Troiani, F.; Affronte, M.; Carretta, S.; Santini, P.; Amoretti, G.
abstract
We propose a scheme for the implementation of quantum gates which is based on the qubit encoding inantiferromagnetic molecular rings.We show that a proper engineering of the intercluster link would resultin an effective coupling that vanishes as far as the system is kept in the computational space, while it isturned on by a selective excitation of specific auxiliary states. These are also shown to allow theperforming of single-qubit and two-qubit gates without an individual addressing of the rings by meansof local magnetic fields.
2005
- Spin-enhanced magnetocaloric effect in molecular nanomagnets
[Articolo su rivista]
Evangelisti, M; Candini, A; Ghirri, A; Affronte, Marco; Brechin, Ek; Mcinnes, Ejl
abstract
An unusually large magnetocaloric effect for the temperature region below 10 K is found for the Fe-14 molecular nanomagnet. This is to large extent caused by its extremely large spin S ground state combined with an excess of entropy arising from the presence of low-lying excited S states. We also show that the highly symmetric Fe-14 cluster core, resulting in small cluster magnetic anisotropy, enables the occurrence of long-range antiferromagnetic order below T-N=1.87 K.
2005
- Topology and spin dynamics in magnetic molecules
[Articolo su rivista]
Carretta, S; Santini, P; Amoretti, G; Affronte, Marco; Ghirri, A; Sheikin, I; Piligkos, S; Timco, G; Winpenny, Rep
abstract
We investigate the role of topology and distortions in the quantum dynamics of magnetic molecules, using a cyclic spin system as reference. We consider three variants of an antiferromagnetic molecular ring, i.e., Cr-8, Cr7Zn, and Cr7Ni, characterized by low-lying states with different total spin S. We theoretically and experimentally study the low-temperature behavior of the magnetic torque as a function of the applied magnetic field. Near level crossings, this observable selectively probes quantum fluctuations of the total spin (S mixing) induced by lowering of the ideal ring symmetry. We show that while a typical distortion of a model molecular structure is very ineffective in opening new S mixing channels, the spin topology is a major ingredient to control the degree of S mixing. This conclusion is further substantiated by low-temperature heat capacity measurements.
2005
- Unusual e(g) 3d x(2)-y(2) orbital ordering and low-energy excitations in the CE structure of NaMn7O12
[Articolo su rivista]
Gauzzi, A; Gilioli, E; Prodi, A; Bolzoni, F; Licci, F; Marezio, M; Calestani, Gl; Affronte, Marco; Huang, Q; Santoro, A; Lynn, J.
abstract
Similarly to the half-doped manganese oxides with perovskite structure like La0.5Ca0.5MnO3. the double perovskite compound NaMn7O12 contains an equal number of Mn3+ and Mn4+ ions in the corner-sharing network of MnO6 octahedra and exhibits a CE order of the charge and spins of these ions at low temperature. Though, in NaMn7O12 the order is complete, the charge and spin ordering transitions are sharp and the system is free of disorder, phase coexistence or structural inhomogeneities thanks to the absence of chemical substitutions. Here we discuss two unusual features of the CE structure of NaMn7O12: (1) the e(g) 3d x(2)-y(2) orbital ordering expected from the direct crystallographic observation of compressed Mn3+O6 octa(2) the existence of a large amount of low-energy hedra below the charge ordering transition. excitations evidenced by the low temperature behavior of the specific heat. We propose a picture of nearly degenerate spin and orbital configurations of the CE structure arising from the peculiar orbital ordering.
2004
- Effects of intercluster coupling in high spin molecular magnets
[Articolo su rivista]
Affronte, Marco; R., Sessoli; D., Gatteschi; W., Wernsdorfer; J. C., Lasjaunias; S. L., Heath; A. K., Powell; A., Fort; A., Rettori
abstract
Effects of intercluster coupling in high spin molecular magnets
2004
- Engineering molecular rings for magnetocaloric effect
[Articolo su rivista]
Affronte, Marco; Ghirri, A; Carretta, S; Amoretti, G; Piligkos, S; Timco, Ga; Winpenny, Rep
abstract
By substituting one Cr3+(s=3/2) with Cd2+(s=0) in molecular octanuclear rings, a diluted ensemble of identical nanomagnets with a S=3/2 ground state, weakly split in zero field, is obtained. The lattice contribution and the essential parameters of the spin Hamiltonian of these uncompensated antiferromagnetic cyclic spin systems are determined by fitting specific heat data between 0.4 and 20 K in magnetic fields up to 7 T. Different entropy contributions are evaluated and results suggest a possible way of engineering molecular magnets to exploit low temperature magnetocaloric effect.
2004
- Indication for a chiral phase in the molecular magnetic chain Gd(hfac)(3)NiTiPr by specific heat and mu+SR measurements
[Articolo su rivista]
Lascialfari, A; Ullu, R; Affronte, Marco; Cinti, F; Caneschi, A; Gatteschi, D; Rovai, D; Pini, Mg; Rettori, A.
abstract
Specific heat and muon spin relaxation (mu(+)SR) measurements performed in the molecular magnetic chain Gd(hfac)(3)NiTiPr provide indication for the onset, at T-0 = 2.08 K, of a phase with chiral order in the absence of long-range helical order. Specific heat data (probing the chirality-chirality correlation function) show a peak at T-0 that disappears upon application of a 5 T magnetic field, while mu(+)SR data (probing the spin-spin correlation function) do not present any anomaly at T-0 nor oscillations in the asymmetry curve below T-0.
2004
- Level crossing in a molecular Cr-8 ring
[Articolo su rivista]
Affronte, Marco; Guidi, T; Caciuffo, R; Carretta, S; Amoretti, G; Hinderer, J; Sheikin, I; Smith, Aa; Winpenny, Rep; van Slageren, J; Gatteschi, D.
abstract
The heat capacity of a molecular Cr-8 ring was measured as a function of temperature and magnetic field in order to follow the evolution of the Schottky anomaly through the first (S = 0 to \1, - I >) level crossing occurring at B-c1 = 6.9 T. The results are interpreted within the framework of a spin Hamiltonian approach and they nicely fit the pattern of the energy levels derived by inelastic neutron spectroscopy. At B-c1, the Schottky anomaly almost vanishes suggesting that a true crossing occurs in Cr-8 in contrast to what was observed for ferric wheels.
2004
- Magnetic and electronic properties of Mn4Si7
[Articolo su rivista]
Sulpice, A; Gottlieb, U; Affronte, Marco; Laborde, O.
abstract
We present a systematic study of the magnetization, Hall effect and specific heat on single crystals of Mn4Si7. Curie-Weiss law is observed above 43 K. At low-temperature moments order in an anisotropic helical state and are aligned above 1 T. We observe an anomalous Hall effect in both rho(H) vs. B and in R-H vs. T curves and a field dependence of the low T specific heat due to spin fluctuations. The magnetic moments (p(eff) and p(sat)) are the lowest reported for similar itinerant magnetic systems, this suggests that Mn4Si7 is a good candidate to observe critical quantum fluctuations expected for a marginal Fermi liquid.
2004
- Thin films of sodium-doped lanthanum manganites: role of substrate and thickness on the magnetoresistive response
[Articolo su rivista]
Malavasi, L; Mozzati, Mc; Alessandri, I; Affronte, Marco; Cervetto, V; Azzoni, Cb; Flor, G.
abstract
In this paper, we report the results about the synthesis and characterization of optimally doped La0.85Na0.15MnO3 thin films deposited onto SrTiO3 (100), NdGaO3 (100) and NdGaO3 (110) for thickness ranging from 11 to 82 nm. The effect of the substrate nature and orientation and film thickness was investigated in order to optimize the physical properties of the films. We obtained very smooth films displaying magnetoresistance values greater than 70% for temperature near to room temperature which makes them appealing for applicative purposes.
2003
- Effects of Al doping on the normal and superconducting properties of MgB2: A specific heat study
[Articolo su rivista]
Putti, M; Affronte, Marco; Manfrinetti, P; Palenzona, A.
abstract
Effects of Al doping on the normal and superconducting properties of MgB2: a specific heat study.
2003
- Microscopic Spin Hamiltonian of a Cr8 Antiferromagnetic Ring from Inelastic Neutron Scattering
[Articolo su rivista]
S., Carretta; J., VAN SLAGEREN; T., Guidi; E., Liviotti; C., Mondelli; D., Rovai; Cornia, Andrea; A. L., Dearden; F., Carsughi; Affronte, Marco; C. D., Frost; R. E. P., Winpenny; D., Gatteschi; G., Amoretti; R., Caciuffo
abstract
Exchange integrals and single-ion anisotropy parameters of a ring-shaped molecular cluster, comprising eight chromium(III) ions (s = 3/2) were determined by inelastic neutron scattering. Effects due to the mixing of different spin multiplets have been considered. Such effects proved to be important to correctly reproduce the relative intensity of magnetic excitations in the neutron spectra. Evidence of decreasing lifetimes with increasing energy of excited spin states was found. The microscopic picture emerging from neutron spectroscopy was confirmed by the temperature dependence of the heat capacity.
2003
- Mixing of magnetic states in a Cr-8 molecular ring
[Articolo su rivista]
Affronte, Marco; Guidi, T; Caciuffo, R; Carretta, S; Amoretti, G; Hinderer, J; Sheikin, I; Jansen, Agm; Smith, Aa; Winpenny, Rep; van Slageren, J; Gatteschi, D.
abstract
Effects of mixing of magnetic states in a molecular Cr-8 ring were investigated by torque magnetometry and heat capacity measurements. Results are interpreted within the framework of the spin Hamiltonian approach allowing us to fit the pattern of energy levels derived by neutron spectroscopy. The torque signal gives evidence of mixing of states with the same parity while the Schottky anomaly vanishes at B-c1=6.9 T and B-c2=14.0 T, the fields at which the S=\0,0> state crosses S=\1,-1> and S=\1,-1> crosses S=\2,-2>, respectively, showing no repulsion between states with different parity, in contrast to what was observed on ferric wheels.
2003
- Some properties of the phonon spectra of transition metal disilicides VSi2, NbSi2, and TaSi2
[Articolo su rivista]
Laborde, O; Bossy, J; Affronte, Marco; Schober, H; Gottlieb, U.
abstract
The phonon spectra of metallic disilicides VSi2, NbSi2, and TaSi2 have been studied in detail by inelastic neutron scattering at 300 K and specific heat measurements between 10 K and 250 K. The specific heat calculated from the generalised phonon density of states extracted from neutron measurements is in good agreement with the measured lattice contribution to the specific heat. The properties of the phonon spectra are discussed in relation with other data reported for these isostructural and isoelectronic disilicides. (C) 2003 Elsevier Science Ltd. All rights reserved.
2003
- Specific heat and mSR measurements in Gd(hfac)3NITiPr molecular magnetic chains: indications for a chiral phase without long range helical order
[Articolo su rivista]
A., Lascialfari; R., Ullu; Affronte, Marco; F., Cinti; A., Caneschi; D., Gatteschi; D., Rovai; Mg, Pini; A., Rettori
abstract
Low-temperature specific heat C(T) and zero-field muon spin resonance (μ+SR) measurements were performed in Gd(hfac)3NITiPr, a quasi-one-dimensional molecular magnet with competing nearest-neighbor and next-nearest-neighbor intrachain exchange interactions. The specific heat data exhibit a λ peak at T0=2.08±0.01 K that disappears upon the application of a 5 T magnetic field. Conversely, the μ+SR data do not present any anomaly at T≈2 K, proving the lack of divergence of the two-spin correlation function as required for usual three-dimensional long-range helical order. Moreover, no muon spin precession can be evinced from the μ+SR asimmetry curves, thus excluding the presence of a long-range-ordered magnetic lattice. These results provide indications for a low-T phase where chiral order is established in absence of long-range helical order.
2003
- Specific heat and muSR measurements in Gd(hfac)3NITiPr molecular magnetic chains: indications for a chiral phase without long range helical order
[Articolo su rivista]
A., Lascialfari; R., Ullu; Affronte, Marco; F., Cinti; A., Caneschi; D., Gatteschi; D., Rovai; M. G., Pini; A., Rettori
abstract
Specific heat and ?SR measurements in Gd(hfac)3NITiPr molecular magnetic chains: indications for a chiral phase without long range helical order
2002
- Effects of antisymmetric interactions in molecular iron rings
[Articolo su rivista]
Cinti, F; Affronte, Marco; Jansen, Agm
abstract
The level crossing mechanism between the ground and the first excited state of Na:Fe-6 antiferromagnetically coupled iron rings is studied by torque magnetometry down to 40 mK and in magnetic fields up to 28 T. The step width at the crossing field B. assumes a finite value at the lowest temperatures. This fact is ascribed to the presence of level anticrossing, not expected for a ring with axial, i.e. S-6 point group, symmetry. Assuming a reduced symmetry, we revised the model Hamiltonian of such a spin system by introducing a Dzyaloshinsky-Moriya (DM) term and we show, by exact diagonalization, that DM term can account for the mixing of states with different parity. In particular, analytical as well numerical analysis show that the introduction of the DM term may contribute to the broadening of the torque step as well as for the finite energy gap at B, observed by heat capacity in a similar ring Li:Fe-6 as previously reported [9].
2002
- Low temperature specific heat of a Fe12 molecular cluster
[Articolo su rivista]
Affronte, Marco; J. C., Lasjaunias; G. L., Abbati
abstract
Low temperature specific heat of a Fe12 molecular cluster
2002
- Magnetic ordering in a high-spin Fe19 magnetic nanomagnet
[Articolo su rivista]
Affronte, Marco; Lasjaunias, Jc; Wernsdorfer, W.; Sessoli, R.; Gatteschi, D.; Health, S. L.; Fort, A.; Rettori, A.
abstract
Magnetic ordering in a high-spin Fe19 magnetic nanomagnet
2002
- Observation of magnetic level repulsion in Fe6 : Li molecular antiferromagnetic rings
[Articolo su rivista]
Affronte, Marco; Cornia, Andrea; Lascialfari, A; Borsa, F; Gatteschi, D; Hinderer, J; Horvatic, M; Jansen, Agm; Julien, Mh
abstract
Heat capacity (C), magnetic torque, and proton NMR relaxation rate (1/T-1) measurements were performed on Fe6:Li single crystals in order to study the crossings between S = 0 and S = 1 and between S = 1 and S = 2 magnetic states of the molecular rings, at magnetic fields B-c1 = 11.7 T and B-c2 = 22.4 T, respectively. C vs B data at 0.78 K show that the energy gap between two states remains finite at B-c 's (Delta(1)/k(B) = 0.86 K and Delta(2)/k(B) = 2.36 K) thus proving that levels repel each other. The large Delta(1) value may also explain the anomalously large width of the peak in 1/T-1 vs B, around B-c1. This anticrossing, unexpected in a centrosymmetric system, requires a revision of the Hamiltonian.
2001
- High-field Torque Magnetometry for Investigating Magnetic Anisotropy in Mn-12-acetate Nanomagnets
[Articolo su rivista]
Cornia, Andrea; Affronte, Marco; D., Gatteschi; A. G. M., Jansen; A., Caneschi; R., Sessoli
abstract
The single-molecule superparamagnet [Mn12O12(OAc)16(H2O)4].2AcOH.4H2O (Mn-12-acetate) has attracted considerable attention because it shows exceedingly slow paramagnetic relaxation at low temperature. The cluster has S-4 symmetry in the solid state and comprises four Mn(IV) ions (S = 3/2) and eight Mn(III) ions (S = 2) which are magnetically coupled to give an S=10 ground state. The ground manifold is largely split in zero magnetic field and many efforts have been spent to determine the zero-field splitting (zfs) parameters alpha, beta and gamma appearing in the fourth-order spin-Hamiltonian H = alpha Sz^2 + beta Sz^4 + gamma (S+^4 + S-^4) + muB.B.g.S. These are of paramount importance for defining the magnetic anisotropy of the cluster, which in turn determines the slow relaxation of the magnetization and quantum tunneling effects at low temperatures. We want to show that cantilever torque magnetometry in high fields is a suitable technique for determining second- and fourth-order anisotropic contributions in high-spin molecules, such as Mn12-acetate. The main advantage of the method lies in its high sensitivity which allows to use very small single crystals. Torque curves have been recorded at 4.2 K by applying the magnetic field (0-28 T) very close to the ab-plane of the tetragonal unit cell. The zfs parameters obtained by this procedure [alpha = -0.389(5) cm-1 and beta = -8.4(5) x 10^-4 cm-1] are in excellent agreement with those determined by spectroscopic techniques, such as high frequency EPR and inelastic neutron scattering.
2001
- Neutron spectroscopy within the S=5 groud multiplet and low-temperature heat capacity in Fe4 magnetic cluster
[Articolo su rivista]
Amoretti, G.; Carretta, S.; Caciuffo, R.; Casalta, H.; Cornia, A.; Affronte, Marco; Gatteschi, D.
abstract
The transitions within the S=5 ground state of the tetrairon(III) molecular cluster Fe4(OCH3)6(dpm)6 (Hdpm = dipivaloylmethane) have been measured by inelastic neutron scattering. The spectra have been interpreted by means of an effective spin Hamiltonian for an isolated S=5 multiplet, taking into account the presence of three different isomers in the compound. It has been shown that for the two dominant isomers the symmetry of the zero-field splitting tensor is not purely axial, but a nonzero rhombic coefficient E is needed to fit the neutron spectra. The results have been discussed in comparison with recent high-field electron paramagnetic resonance data in single crystals. Heat-capacity measurements in the range 2 to 20 K have been performed and shown to be compatible with the neutron results for the zero-field splitting.
2000
- Low temperature specific heat of molecular rings: a study on the effects of the internal guest substitution and on the lattice contribution
[Articolo su rivista]
Affronte, Marco; J. C., Lasjaunias; Cornia, Andrea
abstract
The effects of the internal guest substitution within hexa-iron molecular rings (Na:Fe6 and Li:Fe6) have been investigated by means of low temperature specific heat (LTSH) measurements. By changing Na to Li as central metal ion, the Schottky anomaly shifts towards lower temperatures. The data analysis is supported by the study of the LTSH of the non-magnetic Na:Ga6 compound which has the same core structure as the iron rings. For the non-magnetic Na:Ga6, significant deviations from the simple C similar to T-3 Debye law were found and the use of conventional C vs. T fitting curves is critically discussed. The singlet-triplet energy gap (E-1/k(B) = 12.1 +/- 0.5 K) and its zero-field splitting (D-1/k(B) = 1.5 +/- 1 K) evaluated from the magnetic contribution of the Li:Fe6 LTSH are compared to values estimated by high field torque magnetometry.
2000
- Low-temperature specific heat of Li : Fe6 molecular magnets
[Articolo su rivista]
Affronte, Marco; Jc, Lasjaunias; Cornia, Andrea
abstract
We have studied the low-temperature (T) specific heat (C) of Li : Fe6 molecular magnets. The Schottky anomaly related to the singlet-triplet energy gap is shifted towards lower temperature as compared with that observed on Na:Fe6. This evidences the effects of the internal-guest substitution in these antiferromagnetically coupled iron(III) rings. The singlet-triplet energy gap (E(1)/k(B) = 14.2 +/- 0.4 K) and its zero-field splitting (D(1)/k(B) = 3.9 +/- 1.0 K) evaluated by the C-versus-T data fitting are compared to the values estimated by the high-field torque experiments. (C) 2000 Elsevier Science B.V, All rights reserved.
2000
- Magnetic Anisotropy of Mn12-acetate Nanomagnets from High-field Torque Magnetometry
[Articolo su rivista]
Cornia, Andrea; Affronte, Marco; A. G. M., Jansen; D., Gatteschi; A., Caneschi; R., Sessoli
abstract
The zero-field splitting (zfs) parameters of the ground S = 10 state in the easy-axis mol. nanomagnet [Mn12O12(OAc)16(H2O)4]·2AcOH·4H2O were detd. by cantilever torque magnetometry (CTM) in high magnetic fields. Torque curves were recorded in the superparamagnetic regime (4.2 K) by applying the magnetic field (0-28 T) very close to the hard magnetic plane of a single-crystal sample. The 2nd- and 4th-order anisotropy parameters obtained by this procedure [α=-0.389(5) cm-1 and β=-8.4(5)×10-4 cm-1 in the spin-Hamiltonian Hzfs = αSz2+βSz4+γ(S+4+S-4)] are in excellent agreement with those detd. by spectroscopic techniques, such as high-frequency EPR and inelastic neutron scattering. High-field CTM provides a simple macroscopic method for studying magnetic anisotropy in high-spin mols.
2000
- New superconducting CaSi2 phase with T-c up to 14K under pressure
[Articolo su rivista]
Affronte, Marco; Sanfilippo, S; Nunez Regueiro, M; Laborde, O; Lefloch, S; Bordet, P; Hanfland, M; Levi, D; Palenzona, A; Olcese, Gl
abstract
Starting from trigonal and semimetallic CaSi2 we have found evidences for a new superconducting phase with onset of T-c up to 14K under pressure. In situ structural investigation shows that trigonal CaSi2 actually transforms to a new phase with hexagonal symmetry at room temperature and approximate to 90 kbar and the lattice parameters dramatically shrink at approximate to 160 kbar, The presence of sp(2) silicon arrangement seems to be the condition promoting superconductivity in this, as well as in other, silicon-based materials. (C) 2000 Elsevier Science B.V. All rights reserved.
2000
- Structural phase transition in CaSi2 under high pressure
[Articolo su rivista]
Bordet, P.; Affronte, Marco; Sanfilippo, S.; NUNEZ REGUEIRO, N.; Laborde, O.; Olcese, G. L.; Palenzona, A.; LE FLOCH, S.; Levy, D.; Hanfland, M.
abstract
Structural phase transition in CaSi2 under high pressure
2000
- Superconductivity with Tc up to 14K in CaSi2 under pressure
[Articolo su rivista]
Sanfilippo, S.; Elsinger, E.; NUNEZ REGUEIRO, N.; Laborde, O.; LE FLOCH, S.; Affronte, Marco; Olcese, G. L.; Palenzona, A.
abstract
We have studied the occurrence of superconductivity under pressure in the CaSi2 family of compounds,namely the semimetallic trigonal phase and the a-ThSi2-type tetragonal one. Although only the latter issuperconducting (Tc51.56 K) at ambient pressure, starting from the trigonal phase and applying high pressure~.12 GPa! we have found a new superconducting phase with Tconset;14 K, among the highest ever found insilicon based materials.
1999
- High-field Torque Magnetometry on Fe6 and Fe10 Molecular Magnets
[Articolo su rivista]
Cornia, Andrea; A. G. M., Jansen; Affronte, Marco
abstract
The authors have applied torque magnetometry to the study of magnetic anisotropy in six- and ten-membered rings of iron(III) ions. By using magnetic fields up to 23 T, the authors have partially overridden the intramol. antiferromagnetic ordering of the spins (J = 20.0 cm-1 and 9.6 cm-1 in Fe6 and Fe10, resp.) to obtain magnetic ground states with S = 1, 2, 3, etc.,. Torque measurements on microgram single crystals at temps. down to 0.45 K showed that spin-flip transitions are accompanied by sharp variations of magnetic anisotropy. The angular dependence of transition fields provided spectroscopic-quality information about the spin-Hamiltonian parameters in the two compds. By analyzing the field dependence of the torque signal the authors detd. the exchange energies and zero-field splitting parameters of the total-spin multiplets with S ≤ 5 in the Fe10 sample. Torque magnetometry represents a high-sensitivity, powerful technique for studying field-induced level crossing in mol. magnets.
1999
- Low temperature thermodynamic properties of molecular magnetic chains
[Articolo su rivista]
Affronte, Marco; Caneschi, A; Cucci, C; Gatteschi, D; Lasjaunias, Jc; Paulsen, C; Pini, Mg; Rettori, A; Sessoli, R.
abstract
We studied the low-temperature specific heat C(T) and the magnetic susceptibility chi(T) of Gd(hfac)(3)NITiPr, a quasi-one-dimensional (ID) molecular magnetic chain with competing nearest-neighbor (nn) and next-nearest-neighbor (nnn) intrachain exchange interactions. The anomaly observed in C(T) was reproducibly found at T-0=2.09 K, although the C(T) curves measured on different samples showed clear evidence for aging effects. For fresh, high-quality samples, the lambda shape of the anomaly and the negligible latent heat (less than 7 x 10(-3) J/mol) strongly suggest the presence of a phase transition of order higher than the first. In the best sample at temperatures well below T, (0.2-1 K), a linear C-vs-T dependence was found, which we interpreted in the framework of ID quantum free spin-wave theory. Strong aging effects were also found in the magnetic susceptibility X(T) which, for the best and freshest sample, exhibits a very small anomaly close to T-0 (at T-chi = 2.1 K), superimposed to a diverging paramagnetic behavior at lower temperatures. The anomaly at T-0(similar to T-chi) could be due to a transition to three-dimensional (3D) helical long-range order, but this is inconsistent with the observed linear C-vs-T behavior, typical of a 1D magnet with strong intrachain antiferromagnetic interactions, An alternative interpretation, capable of explaining the C(T) data both in the low-temperature and in the critical region, calls for the onset, below T-0, of nonconventional chiral long-range order due to interchain interactions. The chiral phase can be described as a collection of parallel corkscrews, all turning clockwise (or all counterclockwise), whereas their phases are random. An estimation of Tn according to this model and using the exchange constants evaluated from the linear C-vs-T slope supports this interpretation. The chiral order is compatible with the breaking of the chain into finite segments, due to aging effects, while the phase coherence necessary for the onset of 3D helical long-range order is destroyed. In this frame-work. a qualitative account for the very small anomaly observed in the magnetic susceptibility chi(T) can also be given. [S0163-1829 (99)06909-X].
1999
- Low-temperature specific heat of Fe-6 and Fe-10 molecular magnets
[Articolo su rivista]
Affronte, Marco; Lasjaunias, Jc; Cornia, Andrea; Caneschi, A.
abstract
The energy splitting of the low-lying levels has been investigated on two magnetic molecular clusters Fe-6 and Fe-10 by means of low-temperature zero-field specific-heat measurements. Significant deviations from the usual C similar to T-2 law were observed above the maximum of the main Schottky anomalies as a result of nonnegligible contributions from the excited spin states with S>1 and the estimated lattice contributions follow a phenomenological power law C/R similar to T-alpha with alpha-2.7 for both these compounds. The singlet-triplet energy gaps evaluated by the Schottky anomaly, T-0=19.2 K for Fe6 and 4.56 K for Fe-10, are smaller than what we can estimate by a simplified spin-Hamiltonian approach in the strong exchange approximation and using the energy levels obtained by the high-field magnetization and susceptibility measurements. This discrepancy asks for a more complex description of the low-lying states of these molecular clusters, beyond the strong exchange approximation. At very low temperatures T<<1 K, two low-energy Schottky anomalies were also observed in Fe-10, probably due to a small fraction of defected rings or to hyperfine contributions.
1999
- Magnetic Anisotropy of Fe6 and Fe10 Molecular Rings by Cantilever Torque Magnetometry in High Magnetic Fields
[Articolo su rivista]
Cornia, Andrea; A. G. M., Jansen; Affronte, Marco
abstract
We studied the magnetic anisotropy of 2 mol. magnets, Fe6 and Fe10, which comprise 6- and 10-membered rings of antiferromagnetically coupled Fe (III) ions (Si = 5/2), resp. Spin-flip transitions induced by the applied magnetic field (≤23 T) were investigated by cantilever torque magnetometry on microgram single crystals at very low temp. (down to 0.45 K). From the sharp, steplike variations of magnetic anisotropy at the transition fields, we detd. the singlet-triplet energy gap (Δ1) and the axial zero-field splitting parameter (D1) for the triplet state of Fe6 [Δ1 = 15.28(1) cm-1, D1 = 4.32(3) cm-1] and Fe10 [Δ1 = 4.479(4) cm-1, D1 = 2.24(2) cm-1]. By analyzing the addnl. steps obsd. in the Fe10 sample, we evaluated the ΔS and DS parameters for the total-spin multiplets with S ≤5. On the basis of our findings, we discuss the origin of magnetic anisotropy in Fe(III) rings and the application of torque magnetometry to the study of field-induced level crossing in mol. magnets.
1999
- Polymerization and Characterization of 4,4’-bis(alkylsulfanyl)-2,2’-bithiophenes
[Articolo su rivista]
Iarossi, Dario; Mucci, Adele; Schenetti, Luisa; Seeber, Renato; F., Goldoni; Affronte, Marco; Nava, Filippo
abstract
Regioregular polymers poly[4,4'-bis(butylsulfanyl)-2,2'-bithiophene] (P1)and poly[4,4'-bis(octylsulfanyl)-2,2'-bithiophene] (P2) were prepared from 4,4'-bis(butylsulfanyl)- and 4,4'-bis(octylsulfanyl)-2,2'-bithiophene, by oxidative polymerization with FeCl3, and characterized by 1-H, and 13-C NMR, FT-IR, ad UV-vis spectroscopies, atomic force microscopy (AFM) , electrical conductivity, and cyclic voltammetry measurements. They exhibit weight-average molecular weights of 8 and 70 kDa, respectively, and are both readily soluble in CHCl3, CH2Cl2, CS2, toluene, and THF. P1 has a lamellar structure, and P2 is a flexible and compact film that can be easily processed and manipulated. In the neutral state, they show UV-vis absorption maxima at about 470 nm in CHCL3 solution and exhibit marked solvatochromism and thermochromism. Furthermore, thay can be cast from orange-red solution to form a violet film. The p-doping potentials have been determined, and the possibility of electrogenerating corresponding polymers has been checked. First comparative characterizations of the electrogenerated polymers bave been carried out.
1999
- Tuning of Magnetic Anisotropy in Hexairon(III) Rings by Host–Guest Interactions. An Investigation by High–Field Torque Magnetometry
[Articolo su rivista]
Cornia, Andrea; Affronte, Marco; A. G. M., Jansen; G. L., Abbati; D., Gatteschi
abstract
Magnetic anisotropy of the antiferromagnetic iron(III) rings [LiFe6(OMe)12(dbm)6]+ (I) and [NaFe6(OMe)12(pmdbm)6]+ (II), where Hdbm=1,3-diphenyl-1,3-propanedione and Hpmdbm=1,3-di(4-methoxyphenyl)-1,3-propanedione, was investigated. Staircase structures were obsd. on the torque curves of I-PF6 and II-ClO4 and were assigned to spin-level crossings from the spin ground state. The host-guest interactions responsible for the large difference in magnetic coupling also yield a dramatic difference in the magnetic anisotropies.
1998
- Magnetoresistance of RuO2-based thick film resistors
[Articolo su rivista]
Affronte, Marco; Campani, M; Morten, Bruno; Prudenziati, Maria; Laborde, O.
abstract
In a series of samples whose composition was systematically changed, we have studied the magnetoresistance (Delta R/R = [R(B)-R]/R) of RuO2-based thick film resistors (TFR's) in magnetic fields (B) up to 20 Tesla and in a temperature range (1.2K<T<40K) in which their resistance exhibits insulating behavior at the boundary between rite strong and the weak localization. At the higher temperatures, Delta R/R exhibits a positive bump, that does not depend on the RuO2 concentration but it changes with the concentration of magnetic Mn impurities diluted in the glassy matrix. For T less than or equal to 20 K magnetoresistance is entirely negative and it has a quadratic magnetic field dependence at low field. We use the weak localization theory to relate these features of the high temperature magnetoresistance to the composition of TFR's. At low temperature (T<4.2K) the negative magnetoresistance shows some peculiarities. The quadratic term shrinks, within a vanishing magnetic field range and the magnetoresistance linearly increases in a wide range of B. Ar the strongest fields and the lowest temperature Delta R/R shows a tendency to saturation. Although these results do not enable to discriminate among different mechanisms, we note that the low temperature (T<4.2 K) behavior is essentially consistent,with recent theory of Nguen et al.
1997
- Low temperature electronic transport in RuO2 based cermet resistors
[Articolo su rivista]
Affronte, Marco; Campani, M; Piccinini, S; Tamborin, M; Morten, Bruno; Prudenziati, Maria; Laborde, O.
abstract
We have studied the temperature (T) dependence of resistance (R) of RuO2-based thick films down to 1.2 K. Samples were prepared from inks containing conductive RuO2 powders (less than or equal to 10% wt.), high lead-silicate glass particles and Mn (less than or equal to 1.4% wt.). We found that the resistance fits the exponential law R=R(0)exp(T-0/T)(x) with x=1/4 and the most resistive samples show a crossover to the x=1/2 regime as the temperature decreases. Both the fitting parameters R-0 and T-0 scale down as the RuO2 fraction increases and they are affected in a similar way by a change of the Mn content. The presence of the two regimes is similar way by a chang of the Mn content. The presence of the two regimes is similar to that observed in n-type GaAs [Phys. Rev. B 39, 8059 (1989)] and n-type CdSe [Phys. Rev. Lett. 64, 2687 (1990)] whilst it disagrees with the behaviour predicted for grain to grain hopping [Phys. Rev. B 27, 2583 (1983)] and with that expected for resonant tunneling between metallic particles [J. Appl. Phys. 48, 5152 (1977)]. We conclude that in our systems the driving charge transport mechanism is electron hopping within the glassy matrix. Since in our case the hopping length is of the same order of the localization length, the puzzling questions arising from our experiments are whether and how the variable range hopping model can be extended beyond its conventional limits.
1997
- Thick film resistors as cryogenic thermometers
[Relazione in Atti di Convegno]
Affronte, Marco; M., Campani; Morten, Bruno; S., Piccinini; M., Prudenziati; M., Tamborin; O., Laborde
abstract
The work was aimed at assessing relatioships betwwen composition and performanceThe temperature dependence of resistance in RuO2-based TFRs was studied in the range 1.2 K to 300 K. The resistance fits the exponential relationship R=R0exp(T/T0)x with x=1/4 at higher temperatures with a transition to the x=1/2 regime on the same sample at lower temperatures (<20 K). The transition temperature Tc between these regimeshas a well defined dependence on the sample resitance Rs. Both the R0 ant T0 values scale down as the RuO2 fraction and show similar dependences on the Mn content. The magnetoresistve responses were measured in same temperature range in magnetic field H uo to 20 Tesla. The measured relative change of resistance never exceedes 0.02 up to 8 Tesla. In summary the developed resistors exhibit superior performances in comparison with the prently used thermometers. In addition they they show predictable properties at changing temperature and magnetic field strength, making easier their calibration.
1996
- Electronic properties of TiSi2 single crystals at low temperatures
[Articolo su rivista]
Affronte, Marco; Laborde, O; Lasjaunias, Jc; Gottlieb, U; Madar, R.
abstract
We report measurements of Hall effect, transverse magnetoresistance, and specific heat on high-quality TiSi2 (C54 phase) single crystals at low temperatures. We used crystals with low residual resistivity (typically rho(4.2 K)=0.15 mu Omega cm) and magnetic fields (B) up to 20 T. These facts allowed us to study the electronic properties from the low (omega(c) tau much less than 1) to the high field regime (omega(c) tau>1, omega(c)=eB/m* being the cyclotron frequency and tau the electron relaxation time) as a function of magnetic-field strength and temperature. The low field Hall coefficient R(H) is negative, almost constant R(H)=-(0.5+/-0.1)x10(-10) m(3)/C between 100 and 300 K and it changes sign at similar to 30 K. The angular dependence of magnetoresistance shows either minima or maxima when the magnetic field is parallel to the principal crystallographic axes. These structures are, however, less pronounced than in other silicides, such as PdSi2 and NbSi2, and this suggests only a weak anisotropy of the TiSi2 Fermi surface. The galvanomagnetic properties behave consistently with band-structure calculations of Mattheiss and Hensel [Phys. Rev. B 39, 7754 (1989)] who found that TiSi2 is a compensated metal with only closed orbits for the Fermi electrons. Using a simple two-band model we estimated, from the low field magnetoresistance, carrier density n(e)=n(h)=(0.45-0.52)x10(22) cm(-3) assuming equal concentration of electrons and holes. Low temperatures (1.6<22 K) specific-heat (C-p) measurements fit a linear C-p/T=gamma+beta T-2 dependence, with gamma=3.35+/-0.05 mJ/K-2 mol and beta=0.0201+/-0.0005 mJ/K-4 mel. From these parameters we estimated the Debye temperature Theta(D)=662+/-4 K and the renormalized electronic density of states at the Fermi surface N(epsilon(F))(1+lambda)=2.85 states/eV cell.
1991
- Hall-effect studies in YBa2Cu3O7/PrBa2Cu3O7 superlattices
[Articolo su rivista]
Affronte, Marco; Triscone, J. M.; Brunner, O.; Antognazza, L.; Miéville, L.; Decroux, M.; Fischer, Ø.
abstract
We measured the resistivity and the Hall coefficient RH in a series of YBa2Cu3O7/PrBa2Cu3O7 multilayers. We found no systematic change of the transport properties with decreasing layer thicknesses down to one unit cell. The resistivity and RH evaluated for the YBa2Cu3O7 layers are slightly higher than in bulk material, suggesting a small decrease of the carrier density in the multilayers. However, we observe no change in the Hall number 1/eRH when the thickness of the YBa1Cu3O7 layers decreases, so that the lowering of Tc observed in the superlattices cannot simply be related to a change in the carrier density. Furthermore, we find that the temperature dependence of RH is very similar to that of bulk materials.
1989
- Out- and in-diffusion of oxygen in YBa2Cu3O7 - x oxide
[Articolo su rivista]
Ottaviani, G.; Nobili, C.; Nava, F.; Affronte, M.; Manfredini, T.; Matacotta, F. C.; Galli, E.
abstract
In situ resistance measurements have been used to investigate the oxygen out- and in-diffusion in YBa2Cu3O7 - x oxide. The oxygen content has been measured by nuclear reaction and by weighing; X-ray diffraction has been used to determine the crystalline structure. Polycrystalline sintered material has been used. Out-diffusion has been investigated by heating samples with x = 0.1 in argon atmosphere at fixed temperature and as a function of the time. The resistance increases exponentially and reaches saturation after 5000 min at 550 °C. The oxygen content, after an initial decrease which occurs in the first 100 min, remains constant. Order-disorder transformation can be responsible for the exponential increase in resistance. The in-diffusion has been studied with a material having x = 0.65 heated in oxygen. As a function of the time the resistance curves continuously decrease and present two slopes: fast at the beginning and slow after a certain time. At first the material is tetragonal and non-superconducting. The sharp decrease in resistivity can be attributed to the presence of a thin continuous conductive skin around the grain or, more likely, to a disorder-order transformation activated by the presence of oxygen and involving a large part of the material. © 1989.
1989
- Oxygen in-diffusion processes in tetragonal YBa2Cu3O7-x oxide
[Articolo su rivista]
Ottaviani, G.; Nobili, C.; Nava, F.; Affronte, M.; Manfredini, T.; Matacotta, F. C.; Galli, E.
abstract
In situ resistance measurements have been used to investigate the oxygen in diffusion in tetragonal YBa2Cu3O7-x oxide. The oxygen content has been measured by nuclear reaction and by weighing; x-ray diffraction has been used to determine the crystalline structure. The polycrystalline bulk material with x=0.65 has been heated in oxygen to 200370°C. As a function of time the resistance curves continuously decrease and present two slopes: fast at the beginning and slow later. At first the material is tetragonal and nonsuperconducting. Complementary weight measurements indicate a significant increase only after a delay, and the oxygen uptake is associated with the slow variation of resistance. The initial slopes of the isothermal resistance versus time curves follow, in an Arrhenius plot, a straight line, suggesting a process with an activation energy of 0.400.05 eV. The sharp decrease in resistivity can be attributed to the presence of a thin continuous conductive skin around the grains or, more likely, to a disorder-order transformation activated by the presence of oxygen and involving a large part of the material. A process having an activation energy of 0.90.1 eV is responsible for the slow variation of the resistance. This is in agreement with published data obtained for oxygen in diffusion in similar conditions. © 1989 The American Physical Society.