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MARCO BELEGGIA

Professore Ordinario
Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica

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Pubblicazioni

2022 - Electron-beam-induced charging of an Al2O3 nanotip studied using off-axis electron holography [Articolo su rivista]
Zheng, F.; Beleggia, M.; Migunov, V.; Pozzi, G.; Dunin-Borkowski, R. E.
abstract

Electrostatic charging of specimens during electron, photon or ion irradiation is a complicated and poorly understood phenomenon, which can affect the acquisition and interpretation of experimental data and alter the functional properties of the constituent materials. It is usually linked to secondary electron emission, but also depends on the geometry and electrical properties of the specimen. Here, we use off-axis electron holography in the transmission electron microscope to study electron-beam-induced charging of an insulating Al2O3 nanotip on a conducting support. The measurements are performed under parallel electron illumination conditions as a function of specimen temperature, electron dose, primary electron energy and surface cleanliness. We observe a lack of reproducibility of charge density measurements after cycling the specimen temperature. Surprisingly, we find both positively and negatively charged regions in closely adjacent parts of the specimen.

2022 - Generation of electron vortex beams with over 1000 orbital angular momentum quanta using a tunable electrostatic spiral phase plate [Articolo su rivista]
Tavabi, A. H.; Rosi, P.; Roncaglia, A.; Rotunno, E.; Beleggia, M.; Lu, P. -H.; Belsito, L.; Pozzi, G.; Frabboni, S.; Tiemeijer, P.; Dunin-Borkowski, R. E.; Grillo, V.
abstract

We report the use of an electrostatic micro-electromechanical systems-based device to produce high quality electron vortex beams with more than 1000 quanta of orbital angular momentum (OAM). Diffraction and off-axis electron holography experiments are used to show that the diameter of the vortex in the diffraction plane increases linearly with OAM, thereby allowing the angular momentum content of the vortex to be calibrated. The realization of electron vortex beams with even larger values of OAM is currently limited by the breakdown voltage of the device. Potential solutions to overcome this problem are discussed.

2022 - In situ off-axis electron holography of real-time dopant diffusion in GaAs nanowires [Articolo su rivista]
Balasubramanian, G. P. S.; Lebedkina, E.; Goktas, N. I.; Wagner, J. B.; Hansen, O.; Lapierre, R.; Semenova, E.; Molhave, K.; Beleggia, M.; Fiordaliso, E. M.
abstract

Off-axis electron holography was used to reveal remote doping in GaAs nanowires occurring during in situ annealing in a transmission electron microscope. Dynamic changes to the electrostatic potential caused by carbon dopant diffusion upon annealing were measured across GaAs nanowires with radial p-p+ core-shell junctions. Electrostatic potential profiles were extracted from holographic phase maps and built-in potentials (Vbi ) and depletion layer widths (DLWs) were estimated as function of temperature over 300-873 K. Simulations in absence of remote doping predict a significant increase of Vbi and DLWs with temperature. In contrast, we measured experimentally a nearly constant Vbi and a weak increase of DLWs. Moreover, we observed the appearance of a depression in the potential profile of the core upon annealing. We attribute these deviations from the predicted behavior to carbon diffusion from the shell to the core through the nanowire sidewalls, i.e. to remote doping, becoming significant at 673 K. The DLW in the p and p+ regions are in the 10-30 nm range.

2021 - Coexistence and Coupling of Multiple Charge Orderings and Spin States in Hexagonal Ferrite [Articolo su rivista]
Cheng, S.; Li, X.; Xu, C.; Liu, Y.; Beleggia, M.; Wu, L.; Wang, W.; Petrovic, C.; Bellaiche, L.; Tao, J.; Zhu, Y.
abstract

The coupling between charge and spin orderings in strongly correlated systems plays a crucial role in fundamental physics and device applications. As a candidate of multiferroic materials, LuFe2O4with a nominal Fe2.5+valence state has the potential for strong charge-spin interactions; however, these interactions have not been fully understood until now. Here, combining complementary characterization methods with theoretical calculations, two types of charge orderings with distinct magnetic properties are revealed. The ground states of LuFe2O4are decided by the parallel/antiparallel coupling of both charge and spin orderings in the adjacent FeO double layers. Whereas the ferroelectric charge ordering remains ferrimagnetic below 230 K, the antiferroelectric ordering undergoes antiferromagnetic-ferrimagnetic-paramagnetic transitions from 2 K to room temperature. This study demonstrates the unique aspects of strong spin-charge coupling within LuFe2O4. Our results shed light on the coexistence and competing nature of orderings in quantum materials.

2021 - Design of a Single Chip, Single Core, ZVS Buck Converter in MHz Domain [Abstract in Atti di Convegno]
Sanusi, B. N.; Beleggia, M.; Ouyang, Z.
abstract

Venturing into MHz switching frequency promotes the integration of inductive DC-DC converter into mobile devices. Combining GaN power switch with low-loss ferrite core materials opens up new possibilities for high-frequency operations. In this work, we investigate the design of a single core inductor and single chip half-bridge switch for ZVS buck converter. Design guidelines are accompanied by preliminary performance assessment via simulations. To demonstrate the performance of our device, we fabricated and tested a 20 W prototype converting 6 - 12 V to 4 V at 2 MHz switching frequency. The prototype achieves η > 90 % at Pout > 15 W and power density of 9.26 W/m2 with < 5 mm thickness.

2021 - Electromagnetic vibration energy harvester with ring magnets [Abstract in Atti di Convegno]
Imbaquingo, C.; Mann, B.; Beleggia, M.; Bahl, C.; Bjork, R.
abstract

This work investigates a one-dimensional electromagnetic vibration energy harvester (EMVEH) that uses magnetic interactions to convert ambient vibration into electrical energy. The prototype consists of four coaxial ring permanent magnets enclosed in a tube. Two of these are fixed at the ends of the tube while the two others are connected via a rod, resembling a dog-bone shape structure, to reduce the friction of the magnets with the inner walls of the tube. The ring magnets are placed so that same polarities are facing each other, resulting in a repulsive force that levitates the dog-bone structure. In this study the repulsive force between the ring magnets and the induced magnetic flux in a set of coils are derived by using a Fourier space approach and the shape function of the magnets. The derived functional expressions for the magnet-magnet and magnet-coil forces are validated by comparison with a finite element model. Then, an experimental prototype of a ring magnet EMVEH is tested and compared with a model evolving the state equations using the derived force expressions, and show that these agree. Finally, the output power of the experimental EMVEH is characterized and shown to be a strong function of the external driving amplitude.

2021 - Investigation of gas-electron interactions with electron holography [Articolo su rivista]
Hyllested, J. AE.; Beleggia, M.
abstract

Using the combination of off-axis electron holography and environmental Transmission Electron Microscopy (TEM), an experimental setup termed 'gas electron holography', we investigate how the presence of gas in the microscope affects the spatial and phase resolution of electron holograms. The gas is introduced either by using an Environmental TEM (ETEM) or a closed-cell holder. The ETEM data on gas electron holography shows that the number of electrons reaching the detector decreases exponentially as a function of gas pressure. From this evidence, we construct a phenomenological model that describes how coherency changes as a function of gas pressure. By linking the model with the concept of inelastic scattering cross section we find that the change in the coherency of the electron beam due to the presence of gas is related to the number of gas molecules present, their atomic weight and the average energy lost due to inelastic scattering. Regarding gas electron holography with a closed cell holder, we conclude that the membranes surrounding the gas are the primary factor in determining the quality of the electron hologram, while the gas pressure inside the cell has a small impact on the spatial and phase resolution of the electron holograms.

2021 - Phase plates in the transmission electron microscope: Operating principles and applications [Articolo su rivista]
Malac, M.; Hettler, S.; Hayashida, M.; Kano, E.; Egerton, R. F.; Beleggia, M.
abstract

In this paper, we review the current state of phase plate imaging in a transmission electron microscope. We focus especially on the hole-free phase plate design, also referred to as the Volta phase plate. We discuss the implementation, operating principles and applications of phase plate imaging. We provide an imaging theory that accounts for inelastic scattering in both the sample and in the hole-free phase plate.

2021 - Simulated clustering dynamics of colloidal magnetic nanoparticles [Articolo su rivista]
Durhuus, F. L.; Wandall, L. H.; Boisen, M. H.; Kure, M.; Beleggia, M.; Frandsen, C.
abstract

Magnetically guided self-assembly of nanoparticles is a promising bottom-up method to fabricate novel materials and superstructures, such as, for example, magnetic nanoparticle clusters for biomedical applications. The existence of assembled structures has been verified by numerous experiments, yet a comprehensive theoretical framework to explore design possibilities and predict emerging properties is missing. Here we present a model of magnetic nanoparticle interactions built upon a Langevin dynamics algorithm to simulate the time evolution and aggregation of colloidal suspensions. We recognise three main aggregation regimes: non-aggregated, linear and clustered. Through systematic simulations we have revealed the link between single particle parameters and which aggregates are formed, both in terms of the three regimes and the chance of finding specific aggregates, which we characterise by nanoparticle arrangement and net magnetic moment. Our findings are shown to agree with past experiments and may serve as a stepping stone to guide the design and interpretation of future studies.

2020 - A New Type of Domain and Interacting Bloch-lines in a Dzyaloshinskii-Moriya Multilayer Thin Film [Articolo su rivista]
Garlow, J.; Pollard, S.; Beleggia, M.; Yang, H.; Zhu, Y.
abstract

2020 - Analytical Force and Flux for a 1-D Electromagnetic Vibration Energy Harvester [Articolo su rivista]
Enrique Imbaquingo, C.; Beleggia, M.; Roberto Insinga, A.; R. H. Bahl, C.; Mann, B.; Bjork, R.
abstract

A vibration energy harvester (VEH) converts the kinetic energy of a moving source into electrical energy. In this article, we consider a 1-D electromagnetic vibration energy harvester (1-D EMVEH) that consists of three coaxial cylindrical permanent magnets enclosed in a tube such that the middle magnet is levitating. The resulting movement of the middle magnet can then induce an electromotive force (EMF) in one or more surrounding coils. Using an analytical model, we derive expressions for the 1-D-EMVEHs characteristic frequency and output power by using the Fourier space approach. First, the magnetostatic energy of the system as a function of the position of the levitating magnet is calculated. Its spatial gradient gives the force acting on a magnet, which drives its dynamics. Next, more accurate magnetic flux and EMF expressions are obtained. The results are compared with experimental measurements, revealing an excellent agreement.

2020 - Electron Holography in Gaseous and Liquid Environment [Articolo su rivista]
Aerøe Hyllested, Jes; Prabhu Sai Balasubramanian, G.; Maria Fiordaliso, Elisabetta; Yesibolati, Murat; Mølhave, Kristian; Beleggia, Marco
abstract

2020 - Electron-Beam Patterning of Vapor-Deposited Solid Anisole [Articolo su rivista]
Zhao, D.; Chang, B.; Beleggia, M.
abstract

The emerging ice lithography (IL) nanofabrication technology differs from conventional electron-beam lithography by working at cryogenic temperatures and using vapor-deposited organic molecules, such as solid water and alkanes, as e-beam resists. In this paper, we systematically investigate e-beam patterning of frozen anisole and assess its performance as an e-beam resist in IL. Dose curves reveal that anisole has a very low contrast of ∼1, with a very weak dependence on primary beam energy in the investigated range of 5-20 keV. The minimum line width of 60 nm is attainable at 20 keV, limited by stage vibration in our apparatus. Notably, various solid states of anisole have been observed and we can control the deposited anisole from crystalline to amorphous state by decreasing the deposition temperature. The critical temperature for forming an amorphous film is 130 K in the vacuum of a microscope chamber. Smooth patterns with a surface roughness of ∼0.7 nm are achieved in the as-deposited amorphous solid anisole. As a proof of principle of 3D fabrication, we finally fabricate nanoscale patterns on exotic silicon micropillars with a high aspect ratio using this resist.

2020 - Mean Inner Potential of Liquid Water [Articolo su rivista]
Yesibolati, M. N.; Lagana, S.; Sun, H.; Beleggia, M.; Kathmann, S. M.; Kasama, T.; Molhave, K.
abstract

Improving our experimental and theoretical knowledge of electric potentials at liquid-solid boundaries is essential to achieve a deeper understanding of the driving forces behind interfacial processes. Electron holography has proved successful in probing solid-solid interfaces but requires knowledge of the materials' mean inner potential (MIP, V0), which is a fundamental bulk material property. Combining off-axis electron holography with liquid phase transmission electron microscopy (LPTEM), we provide the first quantitative MIP determination of liquid water V0=+4.48±0.19 V. This value is larger than most theoretical predictions, and to explain the disagreement we assess the dominant factors needed in quantum simulations of liquid water. A precise MIP lays the foundations for nanoscale holographic potential measurements in liquids, and provides a benchmark to improve quantum mechanical descriptions of aqueous systems and their interfaces in, e.g., electrochemistry, solvation processes, and spectroscopy.

2020 - Measurement of charge density in nanoscale materials using off-axis electron holography [Articolo su rivista]
Zheng, F.; Caron, J.; Migunov, V.; Beleggia, M.; Pozzi, G.; Dunin-Borkowski, R. E.
abstract

Three approaches for the measurement of charge density distributions in nanoscale materials from electron optical phase images recorded using off-axis electron holography are illustrated through the study of an electrically biased needle-shaped sample. We highlight the advantages of using a model-based iterative algorithm, which allows a priori information, such as the shape of the object and the influence of charges that are located outside the field of view, to be taken into account. The recovered charge density can be used to infer the electric field and electrostatic potential.

2020 - Quantitative measurement of charge accumulation along a quasi-one-dimensional W5O14 nanowire during electron field emission [Articolo su rivista]
Zheng, F.; Pozzi, G.; Migunov, V.; Pirker, L.; Remskar, M.; Beleggia, M.; Dunin-Borkowski, R. E.
abstract

We use an electron holographic method to determine the charge distribution along a quasi-one-dimensional W5O14 nanowire during in situ field emission in a transmission electron microscope. The results show that the continuous charge distribution along the nanowire is not linear, but that there is an additional accumulation of charge at its apex. An analytical expression for this additional contribution to the charge distribution is proposed and its effect on the field enhancement factor and emission current is discussed.

2020 - Toward the quantitative the interpretation of hole-free phase plate images in a transmission electron microscope [Articolo su rivista]
Harada, K.; Malac, M.; Hayashida, M.; Niitsu, K.; Shimada, K.; Homeniuk, D.; Beleggia, M.
abstract

We present progress toward the quantitative interpretation of phase contrast images obtained using a hole-free phase plate (HFPP) in a transmission electron microscope (TEM). We consider a sinusoidal phase grating test object composed of ~5 nm deep groves in a ~13 nm thick amorphous silicon membrane. The periodic grating splits the beam current into direct beam and diffracted side beams in the focal plane of the imaging lens, where the HFPP is located. The physical separation between the beams allows for a detailed study of the HFPP phase shift evolution and its effect on image contrast. The residual phase shift of the electron beam footprint on the phase plate was measured by electron holography and used as input to image simulations that were compared to experimental data. Our results confirm that phase contrast is established by the phase difference between the direct and side beams, which we can estimate by fitting the image contrast evolution in time with an analytical formula describing the image intensity of a sinusoidal strong phase object. We also observed contrast reversal and frequency doubling of the grating image with time, which we interpret as the phase contrast arising from the interference between side beams becoming dominant. Another observation is the lateral displacement of the image fringes, which can be accounted for by a phase difference between the side beams.

2020 - Unconventional domain-wall pairs and interacting Bloch lines in a Dzyaloshinskii-Moriya multilayer thin film [Articolo su rivista]
Garlow, J. A.; Beleggia, M.; Pollard, S. D.; Yang, H.; Zhu, Y.
abstract

Bloch lines (BLs) play a key role in determining the static and dynamic behavior of chiral domain walls and skyrmions in multilayer films with a significant Dzyaloshinskii-Moriya interaction (DMI). Here, using in situ Lorentz phase microscopy, we reveal a type of spin texture termed a type-II domain-wall pair (DWP), stabilized in DMI multilayer thin films with BLs. For a type-II DWP, the Bloch components of each complementary domain wall are parallel, implying opposite chirality. We find that type-II DWPs preferentially form through bifurcation of a type-I DWP and require the formation of at least two BLs. We demonstrate a distinct phase jump associated with type-II DWPs, and further we reveal the role they play in the formation of mixed-character skyrmions where the Bloch component can be of either left- or right-handed chirality.

2019 - Live Measurement of Electrical Charge Density in Materials using Off-Axis Electron Holography [Articolo su rivista]
Voelkl, E.; Zheng, F.; Migunov, V.; Beleggia, M.; Dunin-Borkowski, R. E.
abstract

2019 - Optical Property-Composition Correlation in Noble Metal Alloy Nanoparticles Studied with EELS [Articolo su rivista]
Kadkhodazadeh, S.; Nugroho, F. A. A.; Langhammer, C.; Beleggia, M.; Wagner, J. B.
abstract

Noble metals are currently the most common building blocks in plasmonics and thus define the available range of optical properties. Their alloying provides a viable strategy to engineer new materials with a tunable range of optical responses. Despite this attractive prospect, the link between composition and optical properties of many noble metal alloys is still not well understood. Here, electron energy-loss spectroscopy is employed to systematically study AuAg and AuPd nanoparticles of varying compositions. The localized surface plasmons, the bulk plasmons, and the permittivity functions of these two sets of alloys are investigated as functions of their composition. In the case of the more widely studied AuAg alloy system, good agreement is found with previous experimental and theoretical studies. The results on the less scrutinized AuPd system provide highly valuable experimental data that complements other experimental investigations and supports the development of theoretical models.

2019 - Quantification of Mixed Bloch-Néel Topological Spin Textures Stabilized by the Dzyaloshinskii-Moriya Interaction in Co/Pd Multilayers [Articolo su rivista]
Garlow, J. A.; Pollard, S. D.; Beleggia, M.; Dutta, T.; Yang, H.; Zhu, Y.
abstract

The three-dimensional structure of nanoscale topological spin textures stabilized by the Dzyaloshinskii-Moriya interaction is governed by the delicate competition between the exchange, demagnetization, and anisotropy energies. The quantification of such spin textures through direct experimental methods is crucial towards understanding the fundamental physics associated with their ordering, as well as their manipulation in spintronic devices. Here, we extend the Lorentz transmission electron microscopy technique to quantify mixed Bloch-Néel chiral spin textures stabilized by the Dzyaloshinskii-Moriya interaction in Co/Pd multilayers. Analysis of the observed intensities under varied imaging conditions coupled to corroborative micromagnetic simulations yields vital parameters that dictate the stability and properties of the complex spin texture, namely, the degree of mixed Bloch-Néel character, the domain wall width, the strength of the Dzyaloshinskii-Moriya interaction, and the exchange stiffness. This approach provides the necessary framework for the application of quantitative Lorentz phase microscopy to a broad array of topological spin systems.

2019 - Quantitative Analysis of Topological, Chiral Spin Textures Stabilized by the Dzyaloshinskii–Moriya Interaction in Co/Pd Multilayers [Articolo su rivista]
Garlow, Joseph A.; Pollard, Shawn D.; Beleggia, Marco; Han, Myung-Geun; Fu, Xuewen; Wu, Lijun; Yang, Hyunsoo; Zhu, Yimei
abstract

2018 - Charging of carbon thin films in scanning and phase-plate transmission electron microscopy [Articolo su rivista]
Hettler, S.; Kano, E.; Dries, M.; Gerthsen, D.; Pfaffmann, L.; Bruns, M.; Beleggia, M.; Malac, M.
abstract

A systematic study on charging of carbon thin films under intense electron-beam irradiation was performed in a transmission electron microscope to identify the underlying physics for the functionality of hole-free phase plates. Thin amorphous carbon films fabricated by different deposition techniques and single-layer graphene were studied. Clean thin films at moderate temperatures show small negative charging while thin films kept at an elevated temperature are stable and not prone to beam-generated charging. The charging is attributed to electron-stimulated desorption (ESD) of chemisorbed water molecules from the thin-film surfaces and an accompanying change of work function. The ESD interpretation is supported by experimental results obtained by electron-energy loss spectroscopy, hole-free phase plate imaging, secondary electron detection and x-ray photoelectron spectroscopy as well as simulations of the electrostatic potential distribution. The described ESD-based model explains previous experimental findings and is of general interest to any phase-related technique in a transmission electron microscope.

2018 - Effect of Molecular Weight on the Feature Size in Organic Ice Resists [Articolo su rivista]
Elsukova, A.; Han, A.; Zhao, D.; Beleggia, M.
abstract

The feature size of patterns obtained by electron-beam lithography (EBL) depends critically on resist properties, beam parameters, development process, and instrument limitations. Frozen layers of simple organic molecules such as n-alkanes behave as negative-tone resists for EBL. With the unique advantage of an in situ thermal treatment replacing chemical development, the entire lithographic process can be performed within a single instrument, thus removing the influence of chemical developers on the feature size. By using an environmental transmission electron microscope, we can also minimize the influence of instrumental limitations and explore the fundamental link between resist characteristics and feature size. Our results reveal that the onset dose of organic ice resists correlates with the inverse molecular weight and that in the thermal development the role of change in solubility of polymers is mirrored in a shift in the solid/vapor critical temperature of organic ices. With a 0.4 pA beam current, we obtained 4.5, 5.5, and 8.5 nm lines with frozen octane, undecane, and tetradecane, respectively, consistent with the predictions of a model we developed that links beam profile and feature size. The knowledge acquired on the response of small organic molecules to electron irradiation, combined with the flexibility and operational advantages of using them as qualified EBL resists, provides us with new opportunities for the design and production of nanodevices and broadens the reach of EBL especially toward biological applications.

2018 - Hole-Free Phase Plate Imaging of a Phase Grating [Articolo su rivista]
Malac, Marek; Hayashida, Misa; Harada, Ken; Shimada, Keiko; Niitsu, Kodai; Rowan, Teddy; Beleggia, Marco
abstract

2018 - Measuring surface charge on a single nanoparticle in liquids using off-axis electron holography [Articolo su rivista]
Yesibolati, M. N.; Kasama, T.; Beleggia, M.; Lagana, S.; Sun, H.; Kathmann, S. M.; Molhave, K.
abstract

2018 - Organic ice resists for 3D electron-beam processing: Instrumentation and operation [Articolo su rivista]
Tiddi, W.; Elsukova, A.; Beleggia, M.; Han, A.
abstract

Organic vapors condensed into thin layers of ice on the surface of a cold substrate are exposed with an electron beam to create resist patterns for lithography applications. The entire spin- and development-free lithography process requires a single custom instrument. We report the design, material choice, implementation and operation of this apparatus. It is based on a scanning electron microscope fitted with an electron beam control system that is normally used for electron beam lithography in a multi-user open-access laboratory. The microscope was also equipped with a gas injection system, a liquid nitrogen cooled cryostage, a temperature control system, and a load-lock. Three steps are required to initialize the apparatus for organic ice resist processing, and two steps are required to restore the apparatus for routine multi-user operations. Five steps are needed to create organic ice resist patterns. Finally, by stacking nanoscale patterns made in organic ice we created 3D structures using two complementary cyclic condense, expose and sublimate processes.

2018 - Tunable Ampere phase plate for low dose imaging of biomolecular complexes [Articolo su rivista]
Tavabi, A. H.; Beleggia, M.; Migunov, V.; Savenko, A.; Oktem, O.; Dunin-Borkowski, R. E.; Pozzi, G.
abstract

A novel device that can be used as a tunable support-free phase plate for transmission electron microscopy of weakly scattering specimens is described. The device relies on the generation of a controlled phase shift by the magnetic field of a segment of current-carrying wire that is oriented parallel or antiparallel to the electron beam. The validity of the concept is established using both experimental electron holographic measurements and a theoretical model based on Ampere's law. Computer simulations are used to illustrate the resulting contrast enhancement for studies of biological cells and macromolecules.

2017 - Computer simulations analysis for determining the polarity of charge generated by high energy electron irradiation of a thin film [Articolo su rivista]
Malac, M.; Hettler, S.; Hayashida, M.; Kawasaki, M.; Konyuba, Y.; Okura, Y.; Iijima, H.; Ishikawa, I.; Beleggia, M.
abstract

Detailed simulations are necessary to correctly interpret the charge polarity of electron beam irradiated thin film patch. Relying on systematic simulations we provide guidelines and movies to interpret experimentally the polarity of the charged area, to be understood as the sign of the electrostatic potential developed under the beam with reference to a ground electrode. We discuss the two methods most frequently used to assess charge polarity: Fresnel imaging of the irradiated area and Thon rings analysis. We also briefly discuss parameter optimization for hole free phase plate (HFPP) imaging. Our results are particularly relevant to understanding contrast of hole-free phase plate imaging and Berriman effect.

2017 - Hole-Free Phase Plate Energy Filtering Imaging of Graphene: Toward Quantitative Hole-Free Phase Plate Imaging in a TEM [Articolo su rivista]
Malac, Marek; Kano, Emi; Hayashida, Misa; Kawasaki, Masahiro; Motoki, Sohei; Egerton, Ray; Ishikawa, Isamu; Okura, Yoshio; Beleggia, Marco
abstract

2017 - Magnetic dipolar ordering and hysteresis of geometrically defined nanoparticle clusters [Articolo su rivista]
Kure, M.; Beleggia, M.; Frandsen, C.
abstract

Magnetic nanoparticle clusters have several biomedical and engineering applications, and revealing the basic interplay between particle configuration and magnetic properties is important for tuning the clusters for specific uses. Here, we consider the nanoparticles as macrospins and use computer simulations to determine their magnetic configuration when placed at the vertices of various polyhedra. We find that magnetic dipoles of equal magnitude arrange in flux-closed vortices on a layer basis, giving the structures a null remanent magnetic moment. Assigning a toroidal moment to each layer, we find that the geometrical arrangement, i.e., "triangular packing" vs. "square packing," of the moments in the adjacent layer determines whether the flux-closed layers are ferrotoroidal (co-rotating vortices) or antiferrotoroidal (counter-rotating vortices). Interestingly, upon adding a single magnetic moment at the center of the polyhedra, the central moment relaxes along one of the principal axes and induces partial alignment of the surrounding moments. The resulting net moment is up to nearly four times that of the single moment added. Furthermore, we model quasi-static hysteresis loops for structures with and without a central moment. We find that a central moment ensures an opening of the hysteresis loop, and the resultant loop areas are typically many-fold larger compared to the same structure without a central moment.

2017 - Organic Ice Resists [Articolo su rivista]
Tiddi, W.; Elsukova, A.; Le, H. T.; Liu, P.; Beleggia, M.; Han, A.
abstract

Electron-beam lithography (EBL) is the backbone technology for patterning nanostructures and manufacturing nanodevices. It involves processing and handling synthetic resins in several steps, each requiring optimization and dedicated instrumentation in cleanroom environments. Here, we show that simple organic molecules, e.g. alcohols, condensed to form thin-films at low temperature demonstrate resist-like capabilities for EBL applications and beyond. The entire lithographic process takes place in a single instrument, and avoids exposing users to chemicals and the need of cleanrooms. Unlike EBL that requires large samples with optically flat surfaces, we patterned on fragile membranes only 5 nm-thin, and 2 × 2 mm2 diamond samples. We created patterns on the nanometer to sub-millimeter scale, as well as three-dimensional structures by stacking layers of frozen organic molecules. Finally, using plasma etching, the organic ice resist (OIR) patterns are used to structure the underlying material, and thus enable nanodevice fabrication.

2017 - The Substrate Effect in Electron Energy-Loss Spectroscopy of Localized Surface Plasmons in Gold and Silver Nanoparticles [Articolo su rivista]
Kadkhodazadeh, S.; Christensen, T.; Beleggia, M.; Mortensen, N. A.; Wagner, J. B.
abstract

Electron energy-loss spectroscopy (EELS) has become increasingly popular for detailed characterization of plasmonic nanostructures, owing to the unparalleled spatial resolution of this technique. The typical setup in EELS requires nanoparticles to be supported on thin substrates. However, as in optical measurements, the substrate material can modify the acquired signal. Here, we have investigated how the EELS signal recorded from supported silver and gold spheroidal nanoparticles at different electron beam impact parameter positions is affected by the choice of a dielectric substrate material and thickness. Consistent with previous optical studies, the presence of a dielectric substrate is found to red-shift localized surface plasmons, increase their line widths, and lead to increased prominence of higher order modes. The extent of these modifications heightens with increasing substrate permittivity and thickness. Specific to EELS, the results highlight the importance of the beam impact parameter and substrate-related Čerenkov losses and charging. Our experimental results are compared with and corroborated by full-wave electromagnetic simulations based on the boundary element method. The results present a comprehensive study of substrate-induced modifications in EELS and allow identification of optimal substrates relevant for EELS studies of plasmonic structures.

2016 - Local charge measurement using off-axis electron holography [Articolo su rivista]
Beleggia, M; Gontard, L C; Dunin-Borkowski, R E
abstract

2016 - Organic ice resist for 2D and 3D patterns by electron beam and extreme ultra violet lithography [Brevetto]
Han, A; Tiddi, W; Beleggia, M
abstract

2015 - Doping GaP Core-Shell Nanowire pn-Junctions: A Study by Off-Axis Electron Holography [Articolo su rivista]
Yazdi, S; Berg, A; Borgstrom, Mt; Kasama, T; Beleggia, M; Samuelson, L; Wagner, Jb
abstract

The doping process in GaP core-shell nanowire pn-junctions using different precursors is evaluated by mapping the nanowires' electrostatic potential distribution by means of off-axis electron holography. Three precursors, triethyltin (TESn), ditertiarybutylselenide, and silane are investigated for n-type doping of nanowire shells; among them, TESn is shown to be the most efficient precursor. Off-axis electron holography reveals higher electrostatic potentials in the regions of nanowire cores grown by the vapor-liquid-solid (VLS) mechanism (axial growth) than the regions grown parasitically by the vapor-solid (VS) mechanism (radial growth), attributed to different incorporation efficiency between VLS and VS of unintentional p-type carbon doping originating from the trimethylgallium precursor. This study shows that off-axis electron holography of doped nanowires is unique in terms of the ability to map the electrostatic potential and thereby the active dopant distribution with high spatial resolution.

2015 - ETEM studies of electrodes and electrocatalysts [Capitolo/Saggio]
Jooss, Ch; Mildner, S; Beleggia, M; Mierwaldt, D; Roddatis, V
abstract

2015 - Effect of Magnetostatic Interactions on Twin Boundary Motion in Ni-Mn-Ga Magnetic Shape Memory Alloy [Articolo su rivista]
Heczko, O; Vokoun, D; Kopecky, V; Beleggia, M
abstract

We investigated the effect of magnetostatic interactions on the field-induced reorientation of martensite variants in Ni50.0Mn27.5Ga22.5. The reorientation, achieved by sweeping a single Type-II twin boundary along the sample, was triggered by a twinning stress of about 0.1 MPa. However, depending on the initial position of the twin boundary, the magnetic field providing the critical stress varied in the range 8-32 kA/m. By taking into account the variants sizes and their mutual interactions, we explained the observed dependence of the switching field on the location of the boundary. The resulting match between model predictions and measurements illustrates the fundamental role played by demagnetization effects and magnetostatic interactions in magnetic shape memory effect.

2015 - Electron Beam-Induced Charging and Modifications of Thin Films [Articolo su rivista]
Malac, M.; Beleggia, M.; Rowan, T.; Egerton, R.; Kawasaki, M.; Okura, Y.; Mcleod, R. A.
abstract

2015 - Environmental TEM Study of Electron Beam Induced Electrochemistry of Pr0.64Ca0.36MnO3 Catalysts for Oxygen Evolution [Articolo su rivista]
Mildner, S; Beleggia, M; Mierwaldt, D; Hansen, Tw; Wagner, Jb; Yazdi, S; Kasama, T; Ciston, J; Zhu, Ym; Jooss, C
abstract

Environmental transmission electron microscopy (ETEVI) studies offer great potential for gathering atomic scale information on the electronic state of electrodes in contact with reactants. It also poses big challenges due to the impact of the high energy electron beam. In this article, we present an ETEM study of a Pr0.64Ca0.36MnO3 (PCMO) thin filth electrocatalySt for water splitting and Oxygen evolution in contact with water vapor: We show by means of off-axis electron holography and electrostatic modeling that the electron beam gives rise to a positive electric sample potential due to secondary electron emission. The value of the electric potential depends on the primary electron flux, the sample's electric Conductivity and grounding, and gas properties. We present evidence that two observed electrochemical reactions are driven by a beam induced electrostatic potential of the order of a volt. The first reaction is an anodic oxidation of oxygen depleted amorphous PCMO which results in recrystallization of the oxide. The Second reaction is oxygen evolution which can be detected by the oxidation of a silane additive and formation of SiO2-gamma at catalytically active surfaces. The quantification of beam induced potentials is an important step for future controlled electrochemical experiments in an ETEM.

2015 - Gas-electron Interaction in the ETEM [Capitolo/Saggio]
Wagner, J B; Beleggia, M
abstract

2015 - Longitudinal domain wall formation in elongated assemblies of ferromagnetic nanoparticles [Articolo su rivista]
Varon, M; Beleggia, M; Jordanovic, J; Schiotz, J; Kasama, T; Puntes, Vf; Frandsen, C
abstract

Through evaporation of dense colloids of ferromagnetic similar to 13 nm epsilon-Co particles onto carbon substrates, anisotropic magnetic dipolar interactions can support formation of elongated particle structures with aggregate thicknesses of 100-400 nm and lengths of up to some hundred microns. Lorenz microscopy and electron holography reveal collective magnetic ordering in these structures. However, in contrast to continuous ferromagnetic thin films of comparable dimensions, domain walls appear preferentially as longitudinal, i.e., oriented parallel to the long axis of the nanoparticle assemblies. We explain this unusual domain structure as the result of dipolar interactions and shape anisotropy, in the absence of inter-particle exchange coupling.

2015 - Simulations of super-structure domain walls in two dimensional assemblies of magnetic nanoparticles [Articolo su rivista]
Jordanovic, J; Beleggia, M; Schiotz, J; Frandsen, C
abstract

We simulate the formation of domain walls in two-dimensional assemblies of magnetic nanoparticles. Particle parameters are chosen to match recent electron holography and Lorentz microscopy studies of almost monodisperse cobalt nanoparticles assembled into regular, elongated lattices. As the particles are small enough to consist of a single magnetic domain each, their magnetic interactions can be described by a spin model in which each particle is assigned a macroscopic "superspin." Thus, the magnetic behaviour of these lattices may be compared to magnetic crystals with nanoparticle superspins taking the role of the atomic spins. The coupling is, however, different. The superspins interact only by dipolar interactions as exchange coupling between individual nanoparticles may be neglected due to interparticle spacing. We observe that it is energetically favorable to introduce domain walls oriented along the long dimension of nanoparticle assemblies rather than along the short dimension. This is unlike what is typically observed in continuous magnetic materials, where the exchange interaction introduces an energetic cost proportional to the area of the domain walls. Structural disorder, which will always be present in realistic assemblies, pins longitudinal domain walls when the external field is reversed, and makes a gradual reversal of the magnetization by migration of longitudinal domain walls possible, in agreement with previous experimental results. (C) 2015 AIP Publishing LLC.

2015 - Towards quantitative electrostatic potential mapping of working semiconductor devices using off-axis electron holography [Articolo su rivista]
Yazdi, S; Kasama, T; Beleggia, M; Yekta, Ms; Mccomb, Dw; Twitchett-Harrison, Ac; Dunin-Borkowski, Re
abstract

Pronounced improvements in the understanding of semiconductor device performance are expected if electrostatic potential distributions can be measured quantitatively and reliably under working conditions with sufficient sensitivity and spatial resolution. Here, we employ off-axis electron holography to characterize an electrically-biased Si p-n junction by measuring its electrostatic potential, electric field and charge density distributions under working conditions. A comparison between experimental electron holographic phase images and images obtained using three-dimensional electrostatic potential simulations highlights several remaining challenges to quantitative analysis. Our results illustrate how the determination of reliable potential distributions from phase images of electrically biased devices requires electrostatic fringing fields, surface charges, specimen preparation damage and the effects of limited spatial resolution to be taken into account. (C) 2014 Elsevier B.V. All rights reserved.

2014 - Charging of phase plates and thin samples under electron beam irradiation [Relazione in Atti di Convegno]
Malac, M; Beleggia, M; Egerton, R F; Kawasaki, M; Bergen, M; Okura, Y; Ishikawa, I; Motoi, K
abstract

2014 - Charging of thin film phase plates under electron beam irradiation [Articolo su rivista]
Malac, M.; Beleggia, M.; Egerton, R.; Kawasaki, M.; Berge, M.; Okura, Y.; Ishikawa, I.; Motoki, K.
abstract

2014 - Evaluation of doping in GaP core-shell nanowire on junction by off-axis electron holography [Articolo su rivista]
Yazdi, S; Berg, A; Kasama, T; Beleggia, M; Borgstrom, T M; Wagner, J
abstract

2014 - Forces between arrays of permanent magnets of basic geometric shapes [Articolo su rivista]
Vokoun, D; Beleggia, M
abstract

We provide formulas for evaluating the magnetic force between two permanent magnet arrays, regularly spaced over a square lattice. We focus on three basic shapes of magnets constituting the arrays: cylinder, sphere and rectangular prism. When the lattice parameter is large, the expressions can be used to calculate the force between two single magnets in a computationally efficient way. The calculations are validated experimentally by measuring the attraction force between two single permanent magnets, where we demonstrate a fair agreement within about 15%. (C) 2013 Elsevier B.V. All rights reserved.

2014 - Interferometric methods for mapping static electric and magnetic fields [Articolo su rivista]
Pozzi, G; Beleggia, M; Kasama, T; Dunin-Borkowski, Re
abstract

The mapping of static electric and magnetic fields using electron probes with a resolution and sensitivity that are sufficient to reveal nanoscale features in materials requires the use of phase-sensitive methods such as the shadow technique, coherent Foucault imaging and the Transport of Intensity Equation. Among these approaches, image-plane off-axis electron holography in the transmission electron microscope has acquired a prominent role thanks to its quantitative capabilities and broad range of applicability. After a brief overview of the main ideas and methods behind field mapping, we focus on theoretical models that form the basis of the quantitative interpretation of electron holographic data. We review the application of electron holography to a variety of samples (including electric fields associated with p-n junctions in semiconductors, quantized magnetic flux in superconductors and magnetization topographies in nanoparticles and other magnetic materials) and electron-optical geometries (including multiple biprism, amplitude and mixed-type set-ups). We conclude by highlighting the emerging perspectives of (i) three-dimensional field mapping using electron holographic tomography and (ii) the model-independent determination of the locations and magnitudes of field sources (electric charges and magnetic dipoles) directly from electron holographic data. (C) 2014 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

2014 - Magnetic imaging with a novel hole-free phase plate [Articolo su rivista]
Pollard, S; Malac, M; Beleggia, M; Kawasaki, M; Zhu, Y
abstract

2014 - Measuring magnetic correlations in nanoparticle assemblies [Articolo su rivista]
Beleggia, M; Frandsen, C
abstract

We illustrate how to extract correlations between magnetic moments in assemblies of nanoparticles from, e. g., electron holography data providing the combined knowledge of particle size distribution, inter-particle distances, and magnitude and orientation of each magnetic moment within a nanoparticle superstructure, We show, based on simulated data, how to build a radial/angular pair distribution function f(r, theta) encoding the spatial and angular difference between every pair of magnetic moments. A scatter-plot of f(r, theta) reveals the degree of structural and magnetic order present, and hence provides a measure of the strength and range of magnetic correlations.

2014 - Quiet magnetic Velcro fastener [Brevetto]
Vokoun, D; Beleggia, M
abstract

2014 - Scaling of the Surface Plasmon Resonance in Gold and Silver Dimers Probed by EELS [Articolo su rivista]
Kadkhodazadeh, S; de Lasson, Jr; Beleggia, M; Kneipp, H; Wagner, Jb; Kneipp, K
abstract

The dependence of surface plasmon coupling on the distance between two nanoparticles (dimer) is the basis of nanometrology tools such as plasmon rulers. Application of these nanometric rulers requires an accurate description of the scaling of the surface plasmon resonance (SPR) wavelength with distance. Here, we have applied electron energy-loss spectroscopy (EELS) and scanning transmission electron I l microscopy (STEM) imaging to investigate the relationship between the SPR wavelength of gold and silver nanosphere dimers (radius R) and interparticle distance (d) in the range 0.1R < d < R. The choice of EELS enables probing the SPRs of E individual dimers, whose dimensions and separation distances are measured in situ with subnanometer resolution using STEM. We find that the decaying exponential description of the fractional SPR wavelength shift with d/2R holds valid only over a limited range of d. instead, within the range 0.1R < d < R the fractional SPR wavelength shift is found to be related to (2R/d)(n), with n similar to 0.9 determined for both gold and silver dimers. Despite this common power dependence, consistently larger SPR wavelength shifts are registered for silver for a given change in d, implying silver dimers to be more sensitive plasmon rulers than their gold counterparts.

2014 - Theoretical study of ferroelectric nanoparticles using phase reconstructed electron microscopy [Articolo su rivista]
Phatak, C; Petford-Long, Ak; Beleggia, M; De Graef, M
abstract

Ferroelectric nanostructures are important for a variety of applications in electronic and electro-optical devices, including nonvolatile memories and thin-film capacitors. These applications involve stability and switching of polarization using external stimuli, such as electric fields. We present a theoretical model describing how the shape of a nanoparticle affects its polarization in the absence of screening charges, and quantify the electron-optical phase shift for detecting ferroelectric signals with phase-sensitive techniques in a transmission electron microscope. We provide an example phase shift computation for a uniformly polarized prolate ellipsoid with varying aspect ratio in the absence of screening charges.

2014 - Towards quantitative off-axis electron holographic mapping of the electric field around the tip of a sharp biased metallic needle [Articolo su rivista]
Beleggia, M; Kasama, T; Larson, Dj; Kelly, Tf; Dunin-Borkowski, Re; Pozzi, G
abstract

We apply off-axis electron holography and Lorentz microscopy in the transmission electron microscope to map the electric field generated by a sharp biased metallic tip. A combination of experimental data and modelling provides quantitative information about the potential and the field around the tip. Close to the tip apex, we measure a maximum field intensity of 82 MV/m, corresponding to a field k factor of 2.5, in excellent agreement with theory. In order to verify the validity of the measurements, we use the inferred charge density distribution in the tip region to generate simulated phase maps and Fresnel (out-of-focus) images for comparison with experimental measurements. While the overall agreement is excellent, the simulations also highlight the presence of an unexpected astigmatic contribution to the intensity in a highly defocused Fresnel image, which is thought to result from the geometry of the applied field. (C) 2014 AIP Publishing LLC.

2013 - Dipolar Magnetism in Ordered and Disordered Low-Dimensional Nanoparticle Assemblies [Articolo su rivista]
Varon, M; Beleggia, M; Kasama, T; Harrison, Rj; Dunin-Borkowski, Re; Puntes, Vf; Frandsen, C
abstract

Magnetostatic (dipolar) interactions between nanoparticles promise to open new ways to design nanocrystalline magnetic materials and devices if the collective magnetic properties can be controlled at the nanoparticle level. Magnetic dipolar interactions are sufficiently strong to sustain magnetic order at ambient temperature in assemblies of closely-spaced nanoparticles with magnetic moments of &gt;= 100 mu(B). Here we use electron holography with sub-particle resolution to reveal the correlation between particle arrangement and magnetic order in self-assembled 1D and quasi-2D arrangements of 15 nm cobalt nanoparticles. In the initial states, we observe dipolar ferromagnetism, antiferromagnetism and local flux closure, depending on the particle arrangement. Surprisingly, after magnetic saturation, measurements and numerical simulations show that overall ferromagnetic order exists in the present nanoparticle assemblies even when their arrangement is completely disordered. Such direct quantification of the correlation between topological and magnetic order is essential for the technological exploitation of magnetic quasi-2D nanoparticle assemblies.

2013 - Electron holography of dipolar magnetism in self-assembled nanoparticle chains [Articolo su rivista]
Beleggia, M; Varon, M; Kasama, T; Dunin-Borkowski, R E; Puntes, V F; Frandsen, C; Harrison, R J
abstract

2013 - Magnetic imaging with a Zernike-type phase plate in a transmission electron microscope [Articolo su rivista]
Pollard, S; Malac, M; Beleggia, M; Kawasaki, M; Zhu, Ym
abstract

We demonstrate the use of a hole-free phase plate (HFPP) for magnetic imaging in transmission electron microscopy by mapping the domain structure in PrDyFeB samples. The HFPP, a Zernike-like imaging method, allows for detecting magnetic signals in-focus to correlate the sample crystal structure and defects with the local magnetization topography, and to evidence stray fields protruding from the sample. Experimental and simulated results are shown and are compared with conventional Fresnel (out-of-focus) images without a phase plate. A key advantage of HFPP imaging is that the technique is free from the reference wave distortion from long-range fields affecting electron holography.

2013 - Phase contrast image simulations for electron holography of magnetic and electric fields [Articolo su rivista]
Beleggia, M; Pozzi, G
abstract

The research on flux line lattices and pancake vortices in superconducting materials, carried out within a long and fruitful collaboration with Akira Tonomura and his group at the Hitachi Advanced Research Laboratory, led us to develop a mathematical framework, based on the reciprocal representation of the magnetic vector potential, that enables us to simulate realistic phase images of fluxons. The aim of this paper is to review the main ideas underpinning our computational framework and the results we have obtained throughout the collaboration. Furthermore, we outline how to generalize the approach to model other samples and structures of interest, in particular thin ferromagnetic films, ferromagnetic nanoparticles and p-n junctions.

2013 - The application of off-axis electron holography to electrically biased single GaN nanowires for electrical resistivity measurements [Articolo su rivista]
Yazdi, S; Kasama, T; Beleggia, M; Chiechonski, C; Kryliouk, O; Wagner, J
abstract

2012 - Convenient contrast enhancement by a hole-free phase plate [Articolo su rivista]
Malac, M; Beleggia, M; Kawasaki, M; Li, P; Egerton, Rf
abstract

Decrease of the irradiation dose needed to obtain a desired signal-to-noise ratio can be achieved by Zernike phase-plate imaging. Here we present results on a hole-free phase plate (HFPP) design that uses the incident electron beam to define the center of the plate, thereby eliminating the need for high precision alignment and with advantages in terms of ease of fabrication. The Zernike-like phase shift is provided by a charge distribution induced by the primary beam, rather than by a hole in the film. Compared to bright-field Fresnel-mode imaging, the hole-free phase plate (HFPP) results in two- to four-fold increase in contrast, leading to a corresponding decrease in the irradiation dose required to obtain a desired signal-to-noise ratio. A local potential distribution, developed due to electron beam-induced secondary-electron emission, is the most likely mechanism responsible for the contrast-transfer properties of the HFPP. Crown (C) 2012 and Elsevier B.V. All rights reserved.

2012 - Forces Between a Permanent Magnet and a Soft Magnetic Plate [Articolo su rivista]
Beleggia, M; Vokoun, D; De Graef, M
abstract

Forces between a hard/permanent magnet of arbitrary shape and an ideally soft magnetic plate in close proximity are derived analytically from the image method applied to magnetostatics. We found that the contact force, defined as the force required to detach the hard magnet from the plate, coincides with that between two identical touching permanent magnets. Furthermore, if the hard and the soft magnets are displaced by some amount, their attraction equals that between two identical permanent magnets displaced by twice that amount. Experimental results are presented that validate the theoretical framework and highlight its limits of applicability.

2012 - Lorentz microscopy and electron holography of magnetic materials [Capitolo/Saggio]
Dunin-Borkowski, R E; Kasama, T; Beleggia, M; Pozzi, G
abstract

2012 - Magnetic guns with cylindrical permanent magnets [Articolo su rivista]
Vokoun, D; Beleggia, M; Heller, L
abstract

The motion of a cylindrical permanent magnet (projectile) inside a tubular permanent magnet, with both magnets magnetized axially, illustrates nicely the physical principles behind the operation of magnetic guns. The force acting upon the projectile is expressed semi-analytically as derivative of the magnetostatic interaction energy. For comparison, the forces involved are also calculated numerically using finite elements methods. Based on the conservation of the magnetostatic and kinetic energies, the exit and asymptotic velocities are determined. The derived formulas can be used to optimize the generated forces and motion of the inner cylindrical magnet. (C) 2011 Elsevier B.V. All rights reserved.

2012 - Practical hole-free phase plate imaging: principles, advantages and pitfalls [Articolo su rivista]
Malac, M; Bergen, M; Beleggia, M; Egerton, R F; Shimizu, M; Furukawa, H
abstract

2011 - Charging of a hole-free thin film phase plate [Brevetto]
Malac, M; Beleggia, M; Kawasaki, M; Egerton, R F
abstract

2011 - Direct measurement of the charge distribution along a biased carbon nanotube bundle using electron holography [Articolo su rivista]
Beleggia, M; Kasama, T; Dunin-Borkowski, Re; Hofmann, S; Pozzi, G
abstract

Nanowires and nanotubes can be examined in the transmission electron microscope under an applied bias. Here we introduce a model-independent method, which allows the charge distribution along a nanowire or nanotube to be measured directly from the Laplacian of an electron holographic phase image. We present results from a biased bundle of carbon nanotubes, in which we show that the charge density increases linearly with distance from its base, reaching a value of similar to 0.8 electrons/nm near its tip. (C) 2011 American Institute of Physics. [doi:10.1063/1.3598468]

2011 - Electron Holography of magnetic materials [Capitolo/Saggio]
Kasama, T; Beleggia, M; Dunin-Borkowski, R E
abstract

2011 - Magnetic forces between arrays of cylindrical permanent magnets [Articolo su rivista]
Vokoun, D; Tomassetti, G; Beleggia, M; Stachiv, I
abstract

Permanent magnet arrays are often employed in a broad range of applications: actuators, sensors, drug targeting and delivery systems, fabrication of self-assembled particles, just to name a few. An estimate of the magnetic forces in play between arrays is required to control devices and fabrication procedures. Here, we introduce analytical expressions for calculating the attraction force between two arrays of cylindrical permanent magnets and compare the predictions with experimental data obtained from force measurements with NdFeB magnets. We show that the difference between predicted and measured force values is less than 10%. (C) 2010 Elsevier B.V. All rights reserved.

2011 - On the magnetostatics of chains of magnetic nanoparticles [Articolo su rivista]
Phatak, C; Pokharel, R; Beleggia, M; De Graef, M
abstract

A novel approach is presented for the computation of the magnetostatic energy of straight and bent chains of identical, uniformly magnetized particles of arbitrary shape. The formalism relies on the concept of the magnetometric tensor field, and allows for closed form expressions for the magnetostatic energy, demagnetization factor, Young's modulus, and bending modulus of chains in terms of the shape amplitude of the particles. Analytical solutions are presented for straight chains of spheres, cubes, and cylinders, and for bent chains of spheres. Numerical results include chains of octahedra, tetrahedra, cuboctahedra, and bi-cones. The axial demagnetization factor for the bi-cone shape is derived in analytical form. An approximate energy expression, using the full shape-dependent interaction formalism for short separation distances, and the standard dipolar interaction expression for larger distances, is introduced. (C) 2011 Elsevier B.V. All rights reserved.

2010 - Convenient contrast enhancement by hole-free phase plate in a TEM [Relazione in Atti di Convegno]
Malac, M; Kawasaki, M; Beleggia, M; Li, P; Egerton, R F
abstract

2010 - Dynamical effects in the study of supported nanocrystals using electron holography [Relazione in Atti di Convegno]
Dunin-Borkowski, R E; Boothroyd, C B; Beleggia, M
abstract

2010 - Dynamical response of catalytic systems in a Cs-corrected environmental transmission electron microscope [Relazione in Atti di Convegno]
Hansen, T W; Dunin-Borkowski, R E; Boothroyd, C B; Beleggia, M; Wagner, J
abstract

2010 - Ferromagnetic domain structures and spin configurations measured in doped manganite [Articolo su rivista]
He, Jq; Volkov, Vv; Beleggia, M; Asaka, T; Tao, J; Schofield, Ma; Zhu, Y
abstract

We report on measurements of the spin configuration across ferromagnetic domains in La(0.325)Pr(0.3)Ca(0.375)MnO(3) films obtained by means of low-temperature Lorentz electron microscopy with in situ magnetizing capabilities. Due to the particular crystal symmetry of the material, we observe two sets of independent ferromagnetic twin variants, one of which was pinned by crystallographic twinning. The spin orientation and configuration of the domains were measured quantitatively using electron diffraction and electron holography, and verified with structural modeling. The observed deviation of spin orientations from the expected 90 degrees head-to-tail configuration across the domain walls was attributed to the variation in the domains' aspect ratios as a result of the demagnetization field. Our study provides important insight on how the spin configuration coupled with the magnetic structure and the crystal symmetry might affect the magnetoresistivity under an applied magnetic field in a strongly correlated electron system.

2010 - Image simulations of kinked vortices for transmission electron microscopy [Articolo su rivista]
Beleggia, M; Pozzi, G; Tonomura, A
abstract

We present an improved model of kinked vortices in high-T-c superconductors suitable for the interpretation of Fresnel or holographic observations carried out with a transmission electron microscope. A kinked vortex is composed of two displaced half-vortices, perpendicular to the film plane, connected by a horizontal flux-line in the plane, resembling a connecting Josephson vortex (JV) segment. Such structures may arise when a magnetic field is applied almost in the plane, and the line tension of the fluxon breaks down under its influence. The existence of kinked vortices was hinted in earlier observations of high-T-c superconducting films, where the Fresnel contrast associated with some vortices showed a dumbbell like appearance. Here, we show that under suitable conditions the JV segment may reveal itself in Fresnel imaging or holographic phase mapping in a transmission electron microscope. (C) 2010 Elsevier B.V. All rights reserved.

2010 - Measurement of the charge distribution along an electrically biased carbon nanotube using electron holography [Relazione in Atti di Convegno]
Beleggia, M; Kasama, T; Dunin-Borkowski, R E; Pozzi, G
abstract

2010 - Near-Curie magnetic anomaly at the Ni/C interface observed by electron holography [Articolo su rivista]
Ferrari, L; Matteucci, G; Schofield, Ma; Beleggia, M; Zhu, Ym
abstract

We analyze with electron holography carried out in a transmission electron microscope the near-Curie behavior of magnetism at the edge of a Nickel thin film coated with Carbon. In-situ experiments with finely controlled variations of the sample temperature reveal an anomaly in the ferromagnetic to paramagnetic phase transition when the film temperature is a few degrees above the nominal Curie point. We interpret the anomaly as a strain-induced spin reorientation transition triggered by the differential thermal expansion of Carbon and Nickel. We present a model that quantitatively reproduces the main features of the observed signal. The model is developed in terms of ananisotropic, temperature dependent exchange coupling between the Nickel moments at the Carbon interface that favors their vertical alignment at low temperatures. (C) 2009 Elseviern B.V. All rights reserved.

2010 - Occurrence of the vortex state for magnetic phase plates [Articolo su rivista]
Hari, Mk; Beleggia, M; Brydson, Rmd
abstract

Tunable magnetic phase plates may be realized as nanorings magnetized in the vortex state, where tunability would be granted by the temperature dependence of the saturation magnetization. Here, we study the statistical occurrence of the magnetic vortex state in circular and pentagonal rings within their magnetic phase diagram. We outline the useful operational range of parameters that may be utilized in practice.

2010 - Phase shift of charged metallic nanoparticles [Articolo su rivista]
Beleggia, M; Pozzi, G
abstract

We calculate the electrostatic potential generated by a charged conducting sphere located at some distance over a conducting substrate by means of two complementary approaches. The potential is then projected along the electron beam direction yielding the electron-optical phase shift. The scenario is compared with a uniformly charged sphere over the same substrate, a model that has been widely employed to interpret phase images of charged particles. We illustrate the implications of our findings in the context of transmission electron microscopy experiments performed on metallic nanoparticles, where this classical analysis can be considered as a useful and insightful starting point towards more accurate, yet more complicate, quantum mechanical approaches. (C) 2009 Elsevier B.V. All rights reserved.

2010 - The influence of magnetostatic interactions in exchange-coupled composite particles [Articolo su rivista]
Vokoun, D; Beleggia, M; De Graef, M; Hou, Hc; Lai, Ch
abstract

Exchange-coupled composite (ECC) particles are the basic constituents of ECC magnetic recording media. We examine and compare two types of ECC particles: (i) core-shell structures, consisting of a hard-magnetic core and a coaxial soft-magnetic shell and (ii) conventional ECC particles, with a hard-magnetic core topped by a soft cylindrical element. The model we present describes the magnetic response of the two ECC particle types, taking into account all significant magnetic contributions to the energy landscape. Special emphasis is given to the magnetostatic (dipolar) interaction energy. We find that both the switching fields and the zero-field energy barrier depend strongly on the particle geometry. A comparison between the two types reveals that core-shell ECC particles are more effective in switching field reduction, while conventional ECC particles maintain a larger overall figure of merit.

2010 - The quantitative measurement of magnetic moments from phase images of nanoparticles and nanostructures-I. Fundamentals [Articolo su rivista]
Beleggia, M; Kasama, T; Dunin-Borkowski, Re
abstract

An approach that can be used to measure the magnetic moment of a magnetized nanoparticle or nanostructure from an electron-optical phase image is introduced. The measurement scheme is based on integration of the gradient of the measured phase image within a circular boundary that contains the structure of interest. The quantity obtained is found to be directly proportional to the magnetic moment of the particle, with a constant of proportionality that does not depend on the particle's shape or magnetization state. The measurement of magnetic moments from both simulated and experimental phase images is demonstrated, and strategies are presented that can be utilized to overcome sources of error associated with, for example, the presence of neighboring magnetic particles and the perturbation of the holographic reference wave. (C) 2009 Elsevier B.V. All rights reserved.

2009 - Demagnetization factors for cylindrical shells and related shapes [Articolo su rivista]
Beleggia, M; Vokoun, D; Graef, M
abstract

Magnetostatic self and interaction energies can be computed via demagnetization factors whenever the magnetic state is close to a uniform state, e. g. in the presence of a strong applied field, or when the dimensions involved are within the single-domain limit. We derive analytical expressions for the demagnetization factors of cylindrical shells and rings with rectangular and square cross-sections. The factors are given either as a combination of elliptic integrals or as a series expansion in powers of the dimensionless ratio between inner and outer radii. Limiting cases are analysed for particular ranges of the shape parameters. We also investigate the ring with a square cross-section, and the elliptic ring, where analytical expressions are provided only for small eccentricity. Finally, we introduce the dipolar coupling integral encoding magnetostatic interactions between a magnetized cylinder and a thin coating on its lateral surface. (C) 2008 Elsevier B.V. All rights reserved.

2009 - General magnetostatic shape-shape interaction forces and torques [Articolo su rivista]
De Graef, M; Beleggia, M
abstract

Expressions for the magnetostatic interaction force and torque between two magnetic objects of arbitrary shape are derived within the shape amplitude formalism. A generalized force is derived as the gradient of the magnetometric tensor field, which is the convolution of the cross-correlation of the object shapes with the dipolar tensor fields. Expressions for the mechanical and magnetic torques are also derived in terms of the magnetometric tensor field. Expressions suitable for numerical evaluation are given as finite Fourier summations. Example computations are given for the interactions between pairs of uniformly magnetized spheres (for which analytical results are compared to numerical results), cubes, octahedra, tetrahedra, and cuboctahedra. The accuracy of the derived numerical relations for energy, force, and torques is of the order of 0.1% for object spacings smaller than the object dimensions. (C) 2009 Elsevier B.V. All rights reserved.

2009 - Magnetostatic interactions and forces between cylindrical permanent magnets [Articolo su rivista]
Vokoun, D; Beleggia, M; Heller, L; Sittner, P
abstract

Permanent magnets of various shapes are often utilized in magnetic actuators, sensors or releasable magnetic fasteners. Knowledge of the magnetic force is required to control devices reliably. Here, we introduce ananalytical expression for calculating the attraction force between two cylindrical permanent magnets on the assumption of uniform magnetization. Although the assumption is not fulfilled exactly in cylindrical magnets, we obtain a very good agreement between the calculated and measured forces between two identical cylindrical magnets and within an array of NdFeB cylindrical magnets. (C) 2009 Elsevier B.V. All rights reserved.

2009 - Magnetostatics of the uniformly polarized torus [Articolo su rivista]
Beleggia, M; De Graef, M; Millev, Yt
abstract

We provide an exhaustive description of the magnetostatics of the uniformly polarized torus and its derivative self-intersecting (spindle) shapes. In the process, two complementary approaches have been implemented, position-space analysis of the Laplace equation with inhomogeneous boundary conditions and a Fourier-space analysis, starting from the determination of the shape amplitude of this topologically non-trivial body. The stray field and the demagnetization tensor have been determined as rapidly converging series of toroidal functions. The single independent demagnetization-tensor eigenvalue has been determined as a function of the unique aspect ratio a of the torus. Throughout the range of values of the ratio, corresponding to a multiply connected torus proper, the axial demagnetization factor N(z) remains close to one half. There is no breach of smoothness of N(z)(alpha) at the topological crossover to a simply connected tight torus (alpha = 1). However, N(z) is a non-monotonic function of the aspect ratio, such that substantially different pairs of corresponding tori would still have the same demagnetization factor. This property does not occur in a simply connected body of the same continuous axial symmetry. Several self-suggesting practical applications are outlined, deriving from the acquired quantitative insight.

2008 - A formula for the image intensity of phase objects in Zernike mode [Articolo su rivista]
Beleggia, M
abstract

I present an analytical expression for the image intensity of a phase object visualized in Zernike phase contrast mode. The formula is valid for periodic and non-periodic weak and strong objects, and accounts for the effects of finite illumination. The expression provided is intended as a generalization of the standard reference formula given in the Born and Wolf [Principles of Optics, sixth ed., Pergamon Press, New York, 1980, p. 427] textbook as well as of the formalism employed to evaluate imaging doses in Zernike mode [M. Malac, M. Beleggia, R. Egerton, Y. Zhu, Ultramicroscopy 108 (2008) 126]. 1 illustrate the usefulness of the improved expression by means of three examples: a sinusoidal phase grating, a Gaussian object, and a phase step. The optimal Zernike phase angle yielding maximum image contrast is found to be object-dependent and not necessarily equal to pi/2. Phase plate optimization criteria are derived and presented for two of the examples considered. (c) 2008 Elsevier B.V. All rights reserved.

2008 - Characterization of JEOL 2100F Lorentz-TEM for low-magnification electron holography and magnetic imaging [Articolo su rivista]
Schofield, Ma; Beleggia, M; Zhu, Y; Pozzi, G
abstract

We present results that characterize the performance and capabilities of the JEOL 2100F-LM electron microscope to carry out holography and quantitative magnetic imaging. We find the microscope is well-suited for studies of magnetic materials, or for semi-conductor dopant profiling, where a large hologram width (similar to 1 mu m) and fine fringe spacing (similar to 1.5 nm) are obtained with good contrast (similar to 20%). We present, as well, measurements of the spherical aberration coefficient C-s=(108.7 +/- 9.6)mm and minimum achievable focal step delta f=(87.6 +/- 1.4) nm for the specialty designed long-focal-length objective lens of this microscope. Further, we detail experiments to accurately measure the optical parameters of the imaging system typical of conventional holography setup in a transmission electron microscope. The role played by astigmatic illumination in the hologram formation is also assessed with a wave-optical model, which we present and discuss. The measurements obtained for our microscope are used to simulate realistic holograms, which we compare directly to experimental holograms finding good agreement. These results indicate the usefulness of measuring these optical parameters to guide the optimization of the experimental setup for a given microscope, and to provide an additional degree of practical experimental possibility. (C) 2007 Published by Elsevier B.V.

2008 - Comment on 'Electron holography on dynamic motion of secondary electrons around sciatic nerve tissues' [Articolo su rivista]
Beleggia, M; Pozzi, G
abstract

We present an alternative interpretation of the holographic phase dislocation loops revealed by Shindo et al. [J. Electron Microsc. 56(1): 1-5 (2007)] around a charged sample. Our interpretation does not involve the motion of secondary electrons around a charged object. It relates, instead, to fluctuating charges on the sample and to the resulting Moire-type patterns in the hologram.

2008 - Imaging of radiation-sensitive samples in transmission electron microscopes equipped with Zernike phase plates [Articolo su rivista]
Malac, M; Beleggia, M; Egerton, R; Zhu, Ym
abstract

We have optimized a bright-field transmission electron microscope for imaging of high-resolution radiation-sensitive materials by calculating the imaging dose no needed to obtain a signal-to-noise ratio (SNR) = 5. Installing a Zernike phase plate (ZP) decreases the dose needed to detect single atoms by as much as a factor of two at 300 kV. For imaging larger objects, such as Gaussian objects with full-width at half-maximum larger than 0.15nm, ZP appears more efficient in reducing the imaging dose than correcting for spherical aberration. The imaging dose no does not decrease with extending of chromatic resolution limit by reducing chromatic aberration, using high accelerating potential (U-0 = 300 kV), because the image contrast increases slower than the reciprocal of detection radius. However, reducing chromatic aberration would allow accelerating potential to be reduced leading to imaging doses below 10e(-)/angstrom(2) for a single iodine atom when a CS-corrector and a ZP are used together. Our simulations indicate that, in addition to microscope hardware, optimization is heavily dependent on the nature of the specimen under investigation. (c) 2007 Elsevier B.V. All rights reserved.

2008 - Low-dose performance of parallel-beam nanodiffraction [Articolo su rivista]
Malac, M; Beleggia, M; Taniguchi, Y; Egerton, Rf; Zhu, Y
abstract

We evaluate the low-dose performance of parallel nano-beam diffraction (NBD) in the transmission electron microscope as a method for characterizing radiation sensitive materials at low electron irradiation dose. A criterion, analogous to Rose's, is established for detecting a diffraction spot with desired signal-to-noise ratio. Our experimental data show that a dose substantially lower than in high-resolution bright-field imaging is sufficient to determine structure and orientation of individual nanoscale objects embedded in amorphous matrix. In an instrument equipped with a cold field-emission gun it is possible to form a probe with sub-3 nm diameter and sub-0.3 mrad convergence angle with sufficient beam current to record a diffraction pattern with less than 0.2 s acquisition time. The interpretation of NBD patterns is identical to that of selected area diffraction patterns. We illustrate the physical principles underlying good low-dose performance of NBD by means of a phase grating. The electron irradiation dose needed to detect a diffraction peak in NBD is found proportional to 1/N(2), where N is the number of lattice planes contributing to the peak. (C) 2008 Elsevier B.V. All rights reserved.

2008 - Retrieving the complex degree of spatial coherence of electron beams [Articolo su rivista]
Carrasquilla-Alvarez, J; Castaneda, R; Garcia-Sucerquia, J; Schofield, Ma; Beleggia, M; Zhu, Y; Matteucci, G
abstract

The possibility to characterize the coherence properties of an electron source is presented. The method, based on the determination of centered-reduced moments of the beam spot, allows the evaluation of both amplitude and phase of the complex degree of spatial coherence. The experimental results are in agreement with a different approach based on the Fourier analysis and with calculations according to the Van Cittert-Zernike theorem. (C) 2006 Published by Elsevier GmbH.

2008 - The two-spin model with dipolar interactions for the exchange coupled composite media [Articolo su rivista]
Vokoun, D; Beleggia, M; Rahman, T; Hou, Hc; Lai, Ch
abstract

We study exchange coupled composite (ECC) media where both the hard and soft coupled layers possess perpendicular anisotropy or one of the layers is superparamagnetic. Our model is used to demonstrate the effect of the dipolar interactions on the coercive field. A series of ECC samples with various thicknesses of Ru spacer is manufactured and analyzed. The structure of the samples prepared is CoPtCr-SiO(2)/Ru/CoPtCr-SiO(2), with the composition varied around that of (CoPt(17)Cr(10))(90)-(SiO(2))(10). The acquired experimental data are used to illustrate the significance of magnetostatic interactions between the two coupled magnetic layers. (C) 2008 American Institute of Physics.

2008 - Vector field electron tomography of magnetic materials: Theoretical development [Articolo su rivista]
Phatak, C; Beleggia, M; De Graef, M
abstract

The theory of vector field electron tomography, the reconstruction of the three-dimensional magnetic induction around a magnetized object, is derived within the framework of Lorentz transmission electron microscopy. The tomographic reconstruction method uses as input two orthogonal tilt series of magnetic phase maps and is based on the vector slice theorem. An analytical reconstruction of the magnetic induction of a single magnetic dipole is presented as a proof-of-concept. The method is compared to two previously reported approaches: a reconstruction starting from the gradient of the magnetic phase maps, and a direct reconstruction of the magnetic vector potential. Numerical examples as well as estimates of the reconstruction errors for a range of magnetic particle shapes are reported. (c) 2007 Elsevier B.V. All rights reserved.

2007 - Bright-field TEM imaging of single molecules: Dream or near future? [Articolo su rivista]
Malac, M; Beleggia, M; Egerton, R; Zhu, Ym
abstract

We examine the suitability of spherical aberration (Cs)-corrected (CS) and uncorrected (UC) transmission electron microscopes (TEM) for conventional bright-field imaging of radiation-sensitive materials. We have chosen an individual molecule suspended in vacuum as a hypothetical example of a well-defined radiation-sensitive sample. We find that for this particular sample, CS instruments provide about 30% improvement over an UC instrument in terms of signal/noise ratio per unit electron dose at 300 kV. The lowest imaging doses can be achieved in CS instruments equipped with high-brightness electron source operated at low incident electron energies. Our calculations suggest that it may be possible to image individual, iodine- or bromine-substituted organic molecules in bright-field mode, at doses lower than the accepted values for radiation damage of aromatic molecules. Crown Copyright (c) 2006 Published by Elsevier B.V. All rights reserved.

2007 - Common reversal mechanisms and correlation between transient domain states and field sweep rate in patterned Permalloy structures [Articolo su rivista]
Lau, Jw; Beleggia, M; Zhu, Y
abstract

Complicated domain configurations generated during the magnetization reversal of Permalloy elements patterned in various shapes are the results of a surprisingly few switching mechanisms. By comparing the results from micromagnetic simulations with the experimentally observed switching events imaged by Lorentz microscopy, we found that nucleation and annihilation of vortices and vortex/antivortex pairs, and the propagation of Neel and cross-tie walls are common mechanisms for magnetization reversal in Permalloy elements with dimensions favorable to domain formation. In addition, for a given element, different mechanism combination(s) may transpire, depending on the sweep rate in the externally applied field. This is because energy maxima and minima are generally field dependent and that different field sweep rate alters the time allotted for thermally assisted transition between states. (C) 2007 American Institute of Physics.

2007 - Phase computations for Electron Holography of nanostructured magnetic materials [Articolo su rivista]
Beleggia, M
abstract

2007 - Polaron melting and ordering as key mechanisms for colossal resistance effects in manganites [Articolo su rivista]
Jooss, C; Wu, L; Beetz, T; Klie, Rf; Beleggia, M; Schofield, Ma; Schramm, S; Hoffmann, J; Zhu, Y
abstract

Polarons, the combined motion of electrons in a cloth of their lattice distortions, are a key transport feature in doped manganites. To develop a profound understanding of the colossal resistance effects induced by external fields, the study of polaron correlations and the resulting collective polaron behavior, i.e., polaron ordering and transition from polaronic transport to metallic transport is essential. We show that static long-range ordering of Jahn-Teller polarons forms a polaron solid which represents a new type of charge and orbital ordered state. The related noncentrosymmetric lattice distortions establish a connection between colossal resistance effects and multiferroic properties, i.e., the coexistence of ferroelectric and antiferromagnetic ordering. Colossal resistance effects due to an electrically induced polaron solid-liquid transition are directly observed in a transmission electron microscope with local electric stimulus applied in situ using a piezo-controlled tip. Our results shed light onto the colossal resistance effects in magnetic field and have a strong impact on the development of correlated electron-device applications such as resistive random access memory (RRAIM).

2007 - Quantitative domain wall width measurement with coherent electrons [Articolo su rivista]
Beleggia, M; Schofield, Ma; Zhu, Y; Pozzi, G
abstract

Quantitative measurements of domain wall widths in a magnetic thin foil of Nd2Fe14B are obtained by the analysis of coherent shadow deformation of the biprism in an electron microscope. Information related to the phase gradient in the direction, perpendicular to the biprism is extracted by comparing recorded images and simulations computed according to the experimental electron-optical configuration and by varying the domain wall width w. We demonstrate the usefulness of the technique for extraction of magnetic information at the nanometer scale. (c) 2006 Elsevier B.V. All rights reserved.

2006 - Demagnetization factors of the general ellipsoid: An alternative to the Maxwell approach [Articolo su rivista]
Beleggia, M; De Graef, M; Millev, Y
abstract

A transparent, exhaustive, and self-contained method for the calculation of the demagnetization tensor of the uniformly magnetized ellipsoid is presented. The method is an alternative to the established Maxwell derivation and is based on a Fourier-space approach to the micromagnetics of magnetized bodies. The key to the success of the procedure lies in the convenient treatment of shape effects through the Fourier representation. The scaled form of the demagnetization factors which depends on two dimensionless aspect ratios is argued to be their natural integral representation. Amongst other advantages, it allows for the immediate implementation of symmetry arguments such that only one of the principal factors needs to be computed. The oblate and prolate ellipsoids of revolution are examined from the same general point of view. The demagnetization factors for these distinct types of spheroid are seen to be related by analytic continuation of well-known Gaussian hypergeometric functions.

2006 - Energy barrier to magnetic vortex nucleation [Articolo su rivista]
Lau, Jw; Bording, Jk; Beleggia, M; Zhu, Y
abstract

An array of micron-sized square Permalloy (TM) elements exhibits pure nucleation behavior under selected ranges of applied field and temperature. As a consequence, macroscopically observed net magnetization loss with logarithm of time can be correlated with discrete vortex nucleation events using in situ Lorentz microscopy. Application of various magnetic fields revealed different resistances to nucleation. Using the Arrhenius-Neel model, the energy barrier to magnetic vortex nucleation is measured and a clear dependence on the applied field is observed. (c) 2006 American Institute of Physics.

2006 - Measurements of functional response of nano-objects using advanced electron microscopy [Relazione in Atti di Convegno]
Zhu, Y; Beetz, T; Wu, L; Klie, R F; Huang, L; Lau, J W; Schofield, M A; Volkov, V V; Beleggia, M; Malac, M
abstract

2006 - Optimized CS-corrected Imaging of Radiation-sensitive High-resolution Objects [Relazione in Atti di Convegno]
Malac, M; Beleggia, M; Egerton, R F; Zhu, Y
abstract

2006 - Phase diagram for magnetic nano-rings [Articolo su rivista]
Beleggia, M; Lau, Jw; Schofield, Ma; Zhu, Y; Tandon, S; De Graef, M
abstract

The minimum-energy single-domain magnetization state in a magnetized nano-ring is determined as a function of material and shape parameters. A phase diagram is derived within the framework of a Fourier-space approach for magnetic computations, showing the expected position of the ground state for any given set of external degrees of freedom. A series of micromagnetic simulations for Suitably chosen parameters, show excellent agreement with the obtained theoretical results. An electron holography experiment has been carried out as a test on phase diagram reliability. The validity of the treatment, in particular the simplification employed in choosing ideal uniform rather than more physical quasi-uniform single-domain states, is thoroughly discussed in order to establish clear boundaries of applicability of the phase diagram. (c) 2005 Elsevier B.V. All rights reserved.

2006 - Quantitative shadow technique for the investigation of magnetic domain wall widths [Articolo su rivista]
Pozzi, G; Beleggia, M; Schofield, Ma; Zhu, Y
abstract

Quantitative measurement of domain wall widths in a magnetic thin foil of Nd2Fe14B and a patterned Ni88Fe12 Permalloy element is obtained by the analysis of coherent shadow deformation of an electrostatic biprism in an electron microscope. Information related to the phase gradient in the direction perpendicular to the biprism is extracted by comparing the recorded shadow images with simulations computed according to the experimental electron optical configuration and by varying a single free parameter: the domain wall width. We demonstrate the feasibility of such experiments and the usefulness of the technique for characterization of magnetic features at different length scales.

2006 - Reversal Behavior of Patterned Ferromagnetic Elements [Relazione in Atti di Convegno]
Zhu, Y; Lau, J W; Beleggia, M; Schofield, M A; Vokov, V V; Malac, M
abstract

2006 - Superparamagnetic behavior of um-sized permalloy disks [Relazione in Atti di Convegno]
Schofield, M A; Beleggia, M; Lau, J W; Zhu, Y
abstract

2006 - Superparamagnetic behavior of μm-sized permalloy disks [Articolo su rivista]
Schofield, M. A.; Beleggia, M.; Lau, J. W.; Zhu, Y.
abstract

2006 - The equivalent ellipsoid of a magnetized body [Articolo su rivista]
Beleggia, M; De Graef, M; Millev, Yt
abstract

The equivalent ellipsoid for magnetized bodies of arbitrary shape can be determined by imposing the equality between the demagnetization factors of the two shapes of equal volume. It is shown that the 'commonsense' criterion for mapping two different shapes by imposing the equality of the demagnetization factors for equal aspect ratios often results in large errors. We propose a general method for the rigorous determination of the equivalent ellipsoid. The cases of the exact equivalent ellipsoids for discs, cylinders with elliptical cross section and prisms are worked out and discussed.

2006 - The fluxgate ring-core demagnetization field [Articolo su rivista]
De Graef, M; Beleggia, M
abstract

The local demagnetization factor for the ring-core flux gate is derived analytically, based on a tangential magnetization model. The results are in good agreement with experimental data for a wide range of ring shape parameters. Approximate expressions in the limit of a narrow, thin ring are obtained, and indicate that the local demagnetization factor scales with the ratio of the cross-sectional area to the total area of the ring. Analytical modeling of the demagnetization factors for a uniform magnetization state results in an underestimate of the local cross-section averaged demagnetization factors by 50% or more. (C) 2006 Elsevier B.V. All rights reserved.

2005 - Cs corrected bright field TEM imaging of radiation sensitive materials [Relazione in Atti di Convegno]
Malac, M; Beleggia, M; Egerton, R F; Zhu, Y
abstract

2005 - Demagnetization factors for elliptic cylinders [Articolo su rivista]
Beleggia, M; De Graef, M; Millev, Yt; Goode, Da; Rowlands, G
abstract

The magnetometric (volume averaged) demagnetization factors for cylinders with elliptical cross section are computed using a Fourier-space approach and compared with similar results obtained with a different treatment. The demagnetization factors are given as a series expansion in the eccentricity epsilon of the elliptical cross section, where the terms up to order epsilon(10) are given explicitly as a function of the cylinder aspect ratio. Other simplified expressions, valid in restricted regimes, are also given. Two different series expansions, obtained previously and valid in particular combinations of shape parameters, are recalled and compared with the new results. After the computation of the magnetostatic and exchange-energy terms associated with a vortex closure-domain state in the elliptic cylinder, the single-domain limit, or the critical size below which the structure can support quasi-uniform magnetization, is derived and discussed.

2005 - Direct correlation of reversal rate dynamics to domain configurations in micron-sized permalloy elements [Articolo su rivista]
Lau, Jw; Beleggia, M; Schofield, Ma; Neumark, Gf; Zhu, Y
abstract

The distribution of states upon the removal of applied magnetic field in an array of 7.5 X 7.5 mu m(2) permalloy square elements, as observed by transmission electron microscopy in Lorentz mode, shows a predominance of two states: the vortex state and the seven-domain state. The distributional dependence of these two states on the rate of change of the reversal field is established. Micromagnetic simulations suggest that vortex nucleation and the subsequent domain-wall propagation are the two primary mechanisms for magnetization reversal. The kinetics of the two pathways is examined in a manner that conforms to the observed distribution of states. (c) 2005 American Institute of Physics.

2005 - Electron Holography and Imaging capabilities of the JEOL 2100F FEG-TEM [Relazione in Atti di Convegno]
Schofield, M A; Beleggia, M; Zhu, Y
abstract

2005 - Electron optical phase-shifts by Fourier methods: Analytical versus numerical calculations [Articolo su rivista]
Fazzini, R; Pozzi, G; Beleggia, M
abstract

The theoretical framework for the computation of electromagnetic fields and electron optical phase-shifts in Fourier space has been recently applied to objects with long-range fringing fields, such as reverse-biased p-n junctions and magnetic stripe domains near a specimen edge. In addition to new analytical results, in this work, we present a critical comparison between numerical and analytical computations. The influence of explicit and implicit boundary conditions on the phase shifts and phase-contrast images is also investigated in detail. (c) 2005 Elsevier B.V. All rights reserved.

2005 - General magnetostatic shape-shape interactions [Articolo su rivista]
Beleggia, M; De Graef, M
abstract

The magnetostatic interaction energy between two magnetic elements of arbitrary,7 Shape is presented as a convolution between the cross-correlation of the particle shapes and the dipolar tensor field. A generalized dipole-dipole interaction is derived, where the magnetic moments associated with the two particles interact through a magnetometric tensor field. carrying all the shape information. Example computations are given in order to verify the correctness of the formalism The well-known result of the interaction between prisms, employed in most micromagnetic simulations. is correctly retrieved. The numerical accuracy of the method is also compared to a simple analytical result. Finally, one additional example computation, two interlaced interacting rings, is presented to show the generality of the formalism. (C) 2004 Elsevier B.V. All rights reserved.

2005 - Magnetic phase imaging with transmission electron microscopy [Capitolo/Saggio]
Beleggia, M; Zhu, Y
abstract

2005 - Phase shift of Reverse Biased p-n Junction Arrays by Fourier Methods [Relazione in Atti di Convegno]
Beleggia, M; Fazzini, P F; Pozzi, G
abstract

2005 - Shape-induced ferromagnetic ordering in a triangular array of magnetized disks [Articolo su rivista]
Beleggia, M; Zhu, Y; Tandon, S; De Graef, M
abstract

A magnetic transition induced by shape anisotropy, geometry, and dipolar interactions has been found in a system of three single domain thin disks. The phase transition occurs only when the disks are in close proximity, and for a narrow range of aspect ratios. Near the transition, the system has an abrupt change from a closure-domain state with zero net magnetization to a magnetized state. The transition can be detected by changes in the hysteresis loops. Micromagnetic simulations with realistic parameters confirm the establishment of ferromagnetic ordering. (C) 2005 American Institute of Physics.

2005 - Theory of Phase Reconstructed Vector Field Electron Tomography [Relazione in Atti di Convegno]
De Graef, M; Beleggia, M
abstract

2004 - A Fourier-space approach for the computation of magnetostatic interactions between arbitrarily shaped particles [Articolo su rivista]
Beleggia, M
abstract

A new formalism has been developed to describe the magnetostatic energy associated with particles of arbitrary shape and magnetization state. The formalism relies on a Fourier space description of the particle shape, through the so-called shape amplitude, which can be used to obtain explicit expressions for the demagnetization tensor field, magnetic field, magnetic induction and magnetostatic energy of a particle for a given magnetization state. Moreover, the interaction energy between particles, located at arbitrary positions in space, which may have different shapes and magnetization states can also be computed. These results may contribute to a deeper understanding of magnetostatic coupling in nanostructures and of the role of shape anisotropy.

2004 - Comparison between discrete and semicontinuous layered models of superconducting vortices in high-Tc materials for TEM observations [Abstract in Atti di Convegno]
Beleggia, M.; Pozzi, G.; Tonomura, A.
abstract

2004 - Detection Limits and Resolution for Quantitative Phase Retrieval through Transport-of-Intensity [Relazione in Atti di Convegno]
Schofield, M A; Beleggia, M; Volkov, V V; Zhu, Y
abstract

2004 - Developing an atomic scale model for grain boundary potentials in perovskite oxides using Z-contrast imaging and EELS [Relazione in Atti di Convegno]
Klie, R; Beleggia, M; Zhu, Y; Buban, J P; Browning, N D
abstract

2004 - Direct evidence for negative grain boundary potential in Ca-doped and undoped YBa2Cu3O7-x [Articolo su rivista]
Schofield, Ma; Beleggia, M; Zhu, Ym; Guth, K; Jooss, C
abstract

Using electron holography in a transmission electron microscope, we obtained direct evidence for the reduction of negative charge at grain boundary dislocations in Ca-doped YBa2Cu3O7 (YBCO) when compared to undoped YBCO. Because of the finite width of the valence band in the superconducting CuO2 planes, the negative grain boundary charge can lead to a depletion of electron holes available for superconductivity. A significant reduction in the size of the perturbed region in the Ca-doped samples appears to be the principal mechanism for the improved interfacial superconductivity.

2004 - Direct measurements of electrostatic potentials at grain boundaries: mechanism for current improvement in high-T-c superconductors [Articolo su rivista]
Jooss, C; Guth, K; Schofield, Ma; Beleggia, M; Zhu, Y
abstract

Grain boundaries (GBs) in high-temperature superconductors represent the major factor limiting high current applications. Via magneto-optical imaging and direct transport measurements, we observed an increase of the intergranular J(c) in 4degrees and 8degrees [001] low-angle grain boundaries via substitution of Y by Ca in YBa2Cu3O7-x by up to 100%. Since Ca substitution leads to hole overdoping, these results suggest that charge depletion and band bending may be responsible for the suppression of superconductivity in GBs. By using electron holography, we directly observe ail electrostatic potential at the dislocation cores. The radius as well as the value of the potential are reduced in Ca-doped grain boundaries, directly reflecting the improved transport properties. (C) 2004 Elsevier B.V. All rights reserved.

2004 - Electron-optical phase shift of a Josephson vortex [Articolo su rivista]
Beleggia, M
abstract

The possibility of directly observing Josephson vortices in a superconducting material by transmission electron microscopy is here investigated. First, the anisotropic London equation for the magnetic field of a Josephson vortex is solved in Fourier space. Then, from the knowledge of the magnetic field, the vector potential and the Aharonov-Bohm phase shift are derived. Finally, phase contrast image simulations are presented. It will be shown that, with current technology, the direct observation of a Josephson vortex is possible.

2004 - Model of superconducting vortices in layered materials for the interpretation of transmission electron microscopy images [Articolo su rivista]
Beleggia, M; Pozzi, G; Tonomura, A; Kasai, H; Matsuda, T; Harada, K; Akashi, T; Masui, T; Tajima, S
abstract

More realistic simulations of the magnetic field and electron optical phase shift associated to pancake vortices in layered high-T-c superconducting specimen require a number of layers larger than 7, the practical upper limit set by the discrete algebraic approach followed so far. This goal can be achieved by resorting to a continuum approximation of the screening layers above and below the one containing the pancake vortex. It is thus possible to increase the number of layers and to investigate more exotic vortex core structures than those represented by the pancakes pinned at tilted columnar defects. In particular it will be shown how recently observed dumbbell-like contrast features in the out-of-focus images of superconducting vortices forming a large angle with the specimen surfaces can be interpreted as due to a kinked structure of the pancakes.

2004 - Modeling superconducting vortices in high-T-c materials for TEM observations [Articolo su rivista]
Beleggia, M; Pozzi, G; Tonomura, A
abstract

In order to improve the model for the interpretation of transmission electron microscopy images of superconducting vortices in layered materials the number of representative layers should be increased. The upper limit of nine layers related to the computer performance has been more than doubled by approximating the screening layers above and below the layer containing a pancake vortex with a superconducting continuum. (C) 2003 Elsevier B.V. All rights reserved.

2004 - Observation of Dipolar Ferromagnetism by TEM techniques [Relazione in Atti di Convegno]
Beleggia, M; Tandon, S; Zhu, Y; De Graef, M
abstract

2004 - On the computation of the demagnetization tensor for particles of arbitrary shape [Articolo su rivista]
Beleggia, M; Tandon, S; Zhu, Ym; De Graef, M
abstract

A new method is presented to compute the demagnetization tensor of particles of arbitrary shape. By means of a Fourier space approach it is possible to compute analytically the Fourier representation of the demagnetization tensor for a broad class of magnetic nanoparticles. Then, specifying the direction of the uniform magnetization, the demagnetizing field and the magnetostatic energy associated with the particle can be evaluated. (C) 2004 Elsevier B.V. All rights reserved.

2004 - On the computation of the demagnetization tensor for uniformly magnetized particles of arbitrary shape. Part I: Analytical approach [Articolo su rivista]
Tandon, S; Beleggia, M; Zhu, Y; De Graef, M
abstract

A Fourier space formalism based on the shape amplitude of a particle is used to compute the demagnetization tensor field for uniformly magnetized particles of arbitrary shape. We provide a list of explicit shape amplitudes for important particle shapes, among others: the sphere, the cylindrical tube, an arbitrary polyhedral shape, a truncated paraboloid, and a cone truncated by a spherical cap. In Part I of this two-part paper, an analytical representation of the demagnetization tensor field for particles with cylindrical symmetry is provided, as well as expressions for the magnetostatic energy and the volumetric demagnetization factors. (C) 2003 Elsevier B.V. All rights reserved.

2004 - On the computation of the demagnetization tensor for uniformly magnetized particles of arbitrary shape. Part II: numerical approach [Articolo su rivista]
Tandon, S; Beleggia, M; Zhu, Y; De Graef, M
abstract

In Part I, we described an analytical approach to the computation of the demagnetization tensor field for a uniformly magnetized particle with an arbitrary shape. In this paper, Part 11, we introduce two methods for the numerical computation of the demagnetization tensor field. One method uses a Fourier space representation of the particle shape, the other starts from the real space representation. The accuracy of the methods is compared to theoretical results for the demagnetization tensor of the uniformly magnetized cylinder with arbitrary aspect ratio. Example computations are presented for the hexagonal plate, the truncated paraboloid, and a so-called "Pac-Man" shape, recently designed for MRAM applications. Finally, the magnetostatic self-energy of a uniformly magnetized regular polygonal disk of arbitrary order is analyzed. A linear relation is found between the order of the polygon and the critical aspect ratio for in-plane vs. axial magnetization states. (C) 2003 Elsevier B.V. All rights reserved.

2004 - On the magnetostatic interactions between nanoparticles of arbitrary shape [Articolo su rivista]
Beleggia, M; Tandon, S; Zhu, Y; De Graef, M
abstract

The general expression for the magnetostatic energy of two magnetized nanoparticles with arbitrary shape and magnetization state is derived within the framework of a Fourier space approach. It is shown how the standard dipole-dipole interaction, valid for large interparticle distances, should be modified in order to take into account the shape anisotropy of each particle. Explicit computations are given for a simple system of two interacting cylinders. For magnetic nanowires, i.e., cylinders with a very large aspect ratio, a simple derivation shows that the interaction is of monopolar, rather than dipolar, nature. (C) 2003 Elsevier B.V. All rights reserved.

2004 - On the transport of intensity technique for phase retrieval [Articolo su rivista]
Beleggia, M; Schofield, Ma; Volkov, Vv; Zhu, Y
abstract

The Transport of Intensity technique is becoming a viable alternative to electron holography for phase retrieval in Transmission Electron Microscopy. However, several issues are still to be clarified in order to ascertain the applicability of the technique; among them, the controversy regarding its geometrical or wave-optical nature, as related to the phase detection limit. We show here that the Transport of Intensity is a wave-optical technique that works in a special regime of small defocus where the image intensity is linear with the defocus parameter. By a simple analytical example we show that the Transport of Intensity correctly reconstructs the electron optical phase shift even when the phase is smaller than pi, a value defining the boundary between the geometrical and wave approaches. Another example is given, the reconstruction of a phase jump, accompanied with experimental support showing that phase retrieval by Electron Holography and Transport of Intensity techniques yields results in good agreement. (C) 2004 Elsevier B.V. All rights reserved.

2003 - A Fourier approach to fields and electron optical phase-shifts calculations [Articolo su rivista]
Beleggia, M; Fazzini, Pf; Pozzi, G
abstract

The Fourier method is applied to calculate fields and electron optical phase shifts in specimens having long-range electromagnetic fields, like reverse biased p-n junctions or stripe magnetic domains. It is shown that this approach not only allows to take into account rather easily the effect of the fringing fields protruding in the space around the specimen, but also to obtain solutions to interesting models in analytical form. (C) 2002 Elsevier Science B.V. All rights reserved.

2003 - Atomic-scale model of the grain boundary potential in perovskite oxides [Articolo su rivista]
Klie, Rf; Beleggia, M; Zhu, Y; Buban, Jp; Browning, Nd
abstract

A combination of experiments and theoretical calculations is used to develop an atomic-scale model of the grain boundary potential in perovskite oxides. More specifically, pristine 8degrees and 58degrees [001] tilt grain boundaries in SrTiO3, which can be regarded as model systems for all cubic perovskite systems, are examined by Z-contrast imaging and electron-energy-loss spectroscopy. Based on results obtained from these systems, distance-valence least-square analysis and multiple-scattering calculations are used to determine the density of grain boundary states for the 8degrees and 58degrees grain boundaries, respectively. To compute the grain boundary potentials, the Thomas-Fermi approach of screened charges and the classical Schottky model is used. The validity of both models for various perovskite oxide grain boundary configurations is discussed, and the appropriate grain boundary potentials are compared with previously reported data.

2003 - Electron Holography and Micromagnetic Simulations for TEM magnetization mapping [Relazione in Atti di Convegno]
Beleggia, M; Schofield, M; Zhu, Y; Malac, M; Liu, Z; Freeman, M
abstract

2003 - Electron-optical phase shift of magnetic nanoparticles I. Basic concepts [Articolo su rivista]
Beleggia, M; Zhu, Y
abstract

The electron-optical phase shift induced in the electron beam due to the interaction with the electromagnetic field of magnetized nanoparticles of defined shape and arbitrary dimensions is calculated, presented and discussed. Together with the computable knowledge of vector potential and magnetic induction, including the demagnetizing field, and with the extension to more realistic geometries which will be presented in part II, this theoretical framework can be employed for the interpretation of transmission electron microscopy experiments on magnetic particles on the nanometre scale.

2003 - Electron-optical phase shift of magnetic nanoparticles II. Polyhedral particles [Articolo su rivista]
Beleggia, M; Zhu, Y; Tandon, S; De Graef, M
abstract

2003 - Influence of charged oxide layers on TEM imaging of reverse-biased p-n junctions [Articolo su rivista]
Beleggia, M; Fazzini, Pf; Merli, Pg; Pozzi, G
abstract

Experimental observations of reverse-biased p-n junctions by means of the out-of-focus method display features which cannot be interpreted within the standard theory of an abrupt p-n junction. In order to reconcile theory and experiment it is necessary to introduce an active role of the specimen surfaces. In particular it is shown how the introduction of a suitable surface density charge at the interface between the silicon and oxide created after the thinning process allows us to explain the main features of the experimental results. Moreover, some questions left unanswered by previous observations made by Lorentz and holographic methods will be clarified. The results point out that oxide charging cannot be overlooked and should be properly taken into account whenever semiconductor devices are observed by transmission electron microscopy techniques, especially when these methods are employed for the analysis of dopant diffusion in submicron devices.

2003 - Measurement of Charge at Grain-Boundary Edge Dislocations in Ca-doped and Undoped YBCO by Electron Holography [Relazione in Atti di Convegno]
Schofield, M A; Beleggia, M; Zhu, Y; Guth, K; Jooss, Ch
abstract

2003 - On the Electron-Optical Phase Shift for Magnetic Nanoparticles [Relazione in Atti di Convegno]
Beleggia, M; Tandon, S; Zhu, Y; De Graef, M
abstract

2003 - On the computation of the demagnetization tensor field for an arbitrary particle shape using a Fourier space approach [Articolo su rivista]
Beleggia, M; De Graef, M
abstract

A method is presented to compute the demagnetization tensor field for uniformly magnetized particles of arbitrary shape. By means of a Fourier space approach it is possible to compute analytically the Fourier representation of the demagnetization tensor field for a given shape. Then, specifying the direction of the uniform magnetization, the demagnetizing field and the magnetostatic energy associated with the particle can be evaluated. In some particular cases, the real space representation is computable analytically. In general, a numerical inverse fast Fourier transform is required to perform the inversion. As an example, the demagnetization tensor field for the tetrahedron will be given. (C) 2003 Elsevier Science B.V. All rights reserved.

2003 - Phase Retrieval and Induction Mapping Artificially Structured Nanometric Magnetic Arrays [Relazione in Atti di Convegno]
Zhu, Y; Volkov, V V; Schofield, M; Beleggia, M; Lau, J; Malac, M
abstract

2003 - Phase retrieval and induction mapping of artificially structured nanometric magnetic arrays [Articolo su rivista]
Zhu, Y.; Volkov, V. V.; Schofield, M.; Beleggia, M.; Lau, J.; Malac, M.
abstract

2003 - Profile structure of magnetic flux lines in type-II superconductor from a rectangular electron hologram [Articolo su rivista]
Harada, K; Beleggia, M; Endo, J; Kasai, H; Togawa, Y; Matsuda, T; Tonomura, A
abstract

Profile magnetic configuration of a quantized flux line and flux-line lattice penetrating a type-II superconductor thin foil, niobium, was observed by electron holography and Lorentz microscopy using a 300 kV field-emission electron microscope. Each single flux line was distributed periodically as lattice structure in the mixed state just below its critical temperature of 8.5 K, while at low temperature of 5 K the flux lines were weakly bound as bundles. in order to observe the flux-line distribution over a wide area, discrete Fourier transform reconstruction in the holography was extended for a rectangular area without loss of information and data precision.

2003 - Quantitative study of magnetic field distribution by electron holography and micromagnetic simulations [Articolo su rivista]
Beleggia, M; Schofield, Ma; Zhu, Y; Malac, M; Liu, Z; Freeman, M
abstract

The magnetic configuration of a submicrometer Ni88Fe12 permalloy island has been quantitatively mapped by off-axis electron holography. The two main contributions to the electron-optical phase shift, namely the phase shifts induced by the electrostatic and magnetic potentials, including fringing fields, were separated by inverting the specimen of 180degrees with respect to the electron beam and directly measuring the mean inner potential. A quantitative map of the projected magnetic induction in the sample was thereby retrieved and compared to results of micromagnetic and electromagnetic calculations, providing the minimum-energy configuration and the phase shift, respectively. (C) 2003 American Institute of Physics.

2002 - Direct evidence of the anisotropic structure of vortices interacting with columnar defects in high-temperature superconductors through the analysis of Lorentz images [Articolo su rivista]
Kamimura, O; Kasai, H; Akashi, T; Matsuda, T; Harada, K; Masuko, J; Yoshida, T; Osakabe, N; Tonomura, A; Beleggia, M; Pozzi, G; Shimoyama, J; Kishio, K; Hanaguri, T; Kitazawa, K; Sasase, M; Okayasu, S
abstract

Two types of Fresnel contrasts of superconducting vortices in a Lorentz micrograph, corresponding to pinned and unpinned vortices, were obtained by a newly developed 1 MV field-emission transmission electron microscope on a Bi2Sr2CaCu2O8+delta (Bi-2212) thin specimen containing tilted linear columnar defects introduced by heavy ion irradiation. The main features of the Fresnel contrasts could be consistently interpreted by assuming that the vortices are pinned along the tilted Columnar defects and by using a layered or an anisotropic model to calculate the phase shift of the electron wave. The confirmed validity of both models strongly indicates that superconducting vortices in high-critical temperature (high-T-c) layered materials have an anisotropic structure.

2002 - Image interpretation of magnetic domains in Nd2Fe14B hard magnets [Relazione in Atti di Convegno]
Beleggia, M; Volkov, V V; Zhu, Y
abstract

2002 - Interpretation of Lorentz microscopy observations of vortices in high-temperature superconductors with columnar defects [Articolo su rivista]
Beleggia, M; Pozzi, G; Masuko, J; Osakabe, N; Harada, K; Yoshida, T; Kamimura, O; Kasai, H; Matsuda, T; Tonomura, A
abstract

In order to interpret recent experimental observations of superconducting vortices interacting with tilted columnar defects in high-temperature superconducting materials, we have extended to the case of anisotropic materials our Fourier space approach for the calculation of the electron optical phase shift experienced by the high-energy electrons in a transmission electron microscope. The case of a London vortex having its core not perpendicular to the specimen surfaces is considered. The same configuration is also analyzed in the framework of a simplified pancake model and the influence of the number of stacks on the phase shift and images is investigated. The results obtained by the two models are compared between them and with the experimental results. The agreement between theory and experiment confirms that anisotropy plays a major role in affecting the electron microscopy images.

2002 - Phase contrast images of superconducting pancake vortices [Relazione in Atti di Convegno]
Beleggia, M; Pozzi, G; Tonomura, A
abstract

2002 - Phase-shift and phase-contrast images of pancake superconducting vortices [Articolo su rivista]
Beleggia, M; Pozzi, G
abstract

Recently, a Fourier space approach has been developed, which can be fruitfully applied for calculating the phase-shift and phase-contrast transmission electron microscope images of superconducting pancake fluxons in high-T-c layered specimens. in this work, the results obtained by a very simplified model, where the specimen is approximated by three thin layers, are presented and discussed. These model calculations, although oversimplified, can nonetheless give useful hints on the expected images of pancake vortices not piercing the specimen but residing in it or near its surfaces, or pinned by a tilted columnar defect. Furthermore, the model calculations motivate more elaborate and accurate, but time and memory consuming, calculations, which can be carried out by increasing the number of layers.

2002 - Vortex modeling in High-Tc anisotropic materials [Relazione in Atti di Convegno]
Beleggia, M; Masuko, J; Osakabe, N; Pozzi, G; Tonomura, A
abstract

2001 - Observation of superconducting fluxons by transmission electron microscopy: A Fourier space approach to calculate the electron optical phase shifts and images [Articolo su rivista]
Beleggia, M; Pozzi, G
abstract

An approach is presented for the calculation of the electron optical phase shift experienced by high-energy electrons in a transmission electron microscope, when they interact with the magnetic field associated with superconducting fluxons in a thin specimen tilted with respect to the beam. It is shown that by decomposing the vector potential in its Fourier components and by calculating the phase shift of each component separately, it is possible to obtain the Fourier transform of the electron optical phase shift, which can be inverted either analytically or numerically. It will be shown how this method can be used to recover the result, previously obtained by the real-space approach, relative to the case of a straight flux tube perpendicular to the specimen surfaces. Then the method is applied to the case of a London fluxon in a thin film, where the bending and the broadening of the magnetic-field lines due to the finite specimen thickness are now correctly taken into account and not treated approximately by means of a parabolic fit. Finally, it will be shown how simple models for the pancake structure of the fluxon can be analyzed within this framework and the main features of electron transmission images predicted.

2000 - A model for the interpretation of holographic and Lorentz images of tilted reverse-biased p-n junctions in a finite specimen [Relazione in Atti di Convegno]
Beleggia, M.; Capelli, R.; Pozzi, G.
abstract

An analytical model for the electric field associated with a periodic array of alternating p- and n-doped stripes lying in a half-plane, tilted with respect to the specimen edges and thus better representing the actual experimental set-up is presented. With respect to a previous treatment, relative to the case of stripes perpendicular to the edge, a more physical derivation is outlined, and the calculated phase shift is used to interpret the main features of holography and Lorentz microscopy images, allowing a quantitative assessment of the influence of the specimen edge on them.

2000 - A model for the interpretation of holographic and Lorentz images of tilted reverse-biased p-n junctions in a finite specimen [Articolo su rivista]
Beleggia, M; Capelli, R; Pozzi, G
abstract

2000 - Electron microscopy of reverse biased p-n junctions [Articolo su rivista]
Beleggia, M; Cristofori, D; Merli, Pg; Pozzi, G
abstract

The aim of this paper is to present and discuss the recent results obtained in the investigation of reverse-biased p-n junctions by means of the out-of-focus method. It will be shown how the interpretation of the experimental images is not in agreement with the expectations based on the bulk p-n junction theory. The possible causes of this discrepancy will be discussed: among them a significant reason could be the finite specimen thickness with the associated surface states, which influences the width and shape of the depletion layer in the thinned specimen. (C) 2000 Elsevier Science Ltd. All rights reserved.

2000 - On the calculation of the phase shift of a superconducting flux line lattice [Relazione in Atti di Convegno]
Beleggia, M; Pozzi, G; Tonomura, A
abstract

2000 - On the calculation of the phase shift of superconducting fluxons: from the isolated to the lattice case [Articolo su rivista]
Beleggia, M; Pozzi, G
abstract

The numerical problems associated to the calculation of the phase shift due to a flux line lattice (FIL) in a thin superconducting specimen are presented and discussed. In order to be able to adequately treat the periodic case, the Fourier transform of the phase shift due to a flux tube is needed. This result is obtained by means of a new approach, where the phase shift is calculated from the beginning in the reciprocal space, treating separately each Fourier coefficient of the magnetic vector potential. The equivalence with the old approach is demonstrated and the utility of the new formulation for calculating the phase shift of closely packed FLL shown. (C) 2000 Elsevier Science B.V. All rights reserved.

1999 - Foucault imaging of superconducting fluxons [Articolo su rivista]
Yoshida, T; Endo, J; Harada, K; Kasai, H; Matsuda, T; Kamimura, O; Tonomura, A; Beleggia, M; Patti, R; Pozzi, G
abstract

The Foucault method, one of the classical Lorentz microscopy techniques, is here investigated in order to demonstrate its applicability to in-focus observations of superconducting fluxons. As the deflections involved are of the order of 10(-5)-10(-6) rad, low-angle techniques, high brightness, coherent illumination, and a low aberration magnetic stage are needed. The first experimental results are presented and discussed in relation with a theoretical model for the fluxon which allows the interpretation of the main features of the observed patterns. (C) 1999 American Institute of Physics. [S0021-8979(99)03708-1].

1999 - Interpretation of holographic and Lorentz images of an array of reverse biased p-n junctions in a semi-infinite specimen [Relazione in Atti di Convegno]
Beleggia, M; Capelli, R; Pozzi, G
abstract

1999 - Recent results in the interpretation of interference and Lorentz images of vortices in superconductors [Articolo su rivista]
Beleggia, M; Fanesi, S; Patti, R; Pozzi, G
abstract

The main ideas and approximations underlying the model developed to interpret the principal features of holographic and Fresnel images of superconducting vortices, or fluxons, are briefly reviewed and applied to the exploitation of new experimental setups, like the standard and coherent Foucault techniques. It is also shown how the model can be implemented to interpret effects due to misalignment and/or distortion of the fluxon cores. Finally, the effect of the finite specimen thickness is also taken into account by resorting to the solution of the Ginzburg-Landau equations found by Clem, and by showing that through it a better agreement can be obtained between theoretical and experimental results. (C) Elsevier Science Inc., 1999. All rights reserved.

1997 - Experiments on Foucault images of superconducting fluxons [Relazione in Atti di Convegno]
Yoshida, T; Endo, J; Harada, K; Kasai, H; Matsuda, T; Beleggia, M; Pozzi, G; Tonomura, A
abstract

1997 - Theoretical Foucault images of superconducting fluxons [Relazione in Atti di Convegno]
Beleggia, M; Pozzi, G; Harada, K; Hasai, H; Matsuda, T; Yoshida, T; Tonomura, A
abstract