[Show abstract][Hide abstract] ABSTRACT: Magnetic properties of newly developed Fe–Si–B–P–Cu nanocrystalline alloys are analyzed by TEM.•Motion of magnetic domain walls were visualized by in-situ Lorentz microscopy.•Borides (Fe2B) in heat-treated specimen cause reduction in domain wall motion smoothness.
Journal of Magnetism and Magnetic Materials 02/2015; 375. · 2.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Electron holographic vector field electron tomography visualized three-dimensional (3D) magnetic vortices in stacked ferromagnetic discs in a nanoscale pillar. A special holder with two sample rotation axes, both without missing-wedges, was used to reduce artifacts in the reconstructed 3D magnetic vectors. A 1 MV holography electron microscope was used for precisely measure the magnetic phase shifts. Comparison of the observed 3D magnetic field vector distributions in the magnetic vortex cores with the results of micromagnetic simulations based on the Landau-Lifshitz-Gilbert equation showed that the proposed technique is well suited for direct 3D visualization of the spin configurations in magnetic materials and spintronics devices.
[Show abstract][Hide abstract] ABSTRACT: This study reports on the correlation between crystal orientation and magnetic flux distribution of Fe3O4 nanoparticles in the form of self-assembled rings. High-resolution transmission electron microscopy demonstrated that the nanoparticles were single-crystalline, highly monodispersed, (25 nm average diameter), and showed no appreciable lattice imperfections such as twins or stacking faults. Electron holography studies of these superparamagnetic nanoparticle rings indicated significant fluctuations in the magnetic flux lines, consistent with variations in the magnetocrystalline anisotropy of the nanoparticles. The observations provide useful information for a deeper understanding of the micromagnetics of ultrasmall nanoparticles, where the magnetic dipolar interaction competes with the magnetic anisotropy.
[Show abstract][Hide abstract] ABSTRACT: Investigation of the effect of electron irradiation on ionic liquid (IL) droplets using electron holography revealed that electron irradiation changed the electrostatic potential around the IL. The potential for low electron flux irradiation (0.5×1017 e/m2s) was almost constant as a function of time (up to 180 min.). For higher electron flux irradiation (2×1017 e/m2s), the potential increased exponentially for a certain time, reflecting the charging effect and then leveled off. The IL was found to be changed from liquid to solid state after a significant increase in the electrostatic potential due to electron irradiation.
[Show abstract][Hide abstract] ABSTRACT: Critical behavior near ferromagnetic transition in EuS and Gd-doped EuS films was studied by magnetization measurement and cryogenic Lorentz microscopy. Ferromagnetic s–f interaction was strongly enhanced by doping 2% Gd. The Curie temperature and critical exponents of the magnetic phase transition for Gd-doped EuS were determined to be 86.3 ± 0.2 K, β = 0.43 ± 0.01, and γ = 1.20 ± 0.05 from the scaling plot, while those for EuS were 14.6 ± 0.1 K, β = 0.39 ± 0.01, and γ = 1.20 ± 0.05. The different universality classes of these materials showed different magnetic domain structures near the Curie temperature: the long-range ferromagnetic ordering based on the mean field model causes the formation of a large domain.
[Show abstract][Hide abstract] ABSTRACT: Direct magnetization measurements from narrow, complex-shaped antiphase boundaries (APBs; that is, planar defect produced in any ordered crystals) are vitally important for advances in materials science and engineering. However, in-depth examination of APBs has been hampered by the lack of experimental tools. Here, based on electron microscopy observations, we report the unusual relationship between APBs and ferromagnetic spin order in Fe70Al30. Thermally induced APBs show a finite width (2-3 nm), within which significant atomic disordering occurs. Electron holography studies revealed an unexpectedly large magnetic flux density at the APBs, amplified by approximately 60% (at 293 K) compared with the matrix value. At elevated temperatures, the specimens showed a peculiar spin texture wherein the ferromagnetic phase was confined within the APB region. These observations demonstrate ferromagnetism stabilized by structural disorder within APBs, which is in direct contrast to the traditional understanding. The results accordingly provide rich conceptual insights for engineering APB-induced phenomena.
[Show abstract][Hide abstract] ABSTRACT: The magnetism of a thin grain-boundary (GB) phase that envelopes the Nd2Fe14B grains in optimally annealed Nd–Fe–B sintered magnets was investigated by electron holography. The phase shift measured from a thin-foil specimen containing a tilted amorphous GB phase (∼3 nm in width) was −0.34 rad, which is substantially smaller than that expected for the nonferromagnetic GB phase of −1.2 rad. Simulations of the phase shift with various magnetization values suggest that the magnetic flux density of the GB phase is ∼1.0 T. The observations imply significant exchange coupling between Nd2Fe14B grains, which can explain the avalanche propagation of magnetization reversal observed in sintered magnets.
[Show abstract][Hide abstract] ABSTRACT: This study demonstrates the accumulation of electron-induced secondary electrons by utilizing a simple geometrical configuration of two branches of a charged insulating biomaterial. The collective motion of these secondary electrons between the branches has been visualized by analyzing the reconstructed amplitude images obtained using in situ electron holography. In order to understand the collective motion of secondary electrons, the trajectories of these electrons around the branches have also been simulated by taking into account the electric field around the charged branches on the basis of Maxwell's equations.
Microscopy and Microanalysis 05/2014; · 1.76 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Skyrmions are nanoscale spin textures that are viewed as promising candidates as information carriers in future spintronic devices. Skyrmions have been observed using neutron scattering and microscopy techniques. Real-space imaging using electrons is a straightforward way to interpret spin configurations by detecting the phase shifts due to electromagnetic fields. Here, we report the first observation by electron holography of the magnetic flux and the three-dimensional spin configuration of a skyrmion lattice in Fe0.5Co0.5Si thin samples. The magnetic flux inside and outside a skyrmion was directly visualized and the handedness of the magnetic flux flow was found to be dependent on the direction of the applied magnetic field. The electron phase shifts φ in the helical and skyrmion phases were determined using samples with a stepped thickness t (from 55 nm to 510 nm), revealing a linear relationship (φ = 0.00173t). The phase measurements were used to estimate the three-dimensional structures of both the helical and skyrmion phases, demonstrating that electron holography is a useful tool for studying complex magnetic structures and for three-dimensional, real-space mapping of magnetic fields.
[Show abstract][Hide abstract] ABSTRACT: Precise evaluation of the electrostatic potential distributions of and around samples with multiple charges using electron holography has long been a problem due to unknown perturbation of the reference wave. Here, we report the first practical application of split-illumination electron holography (SIEH) to tackle this problem. This method enables the use of a non-perturbed reference wave distant from the sample. SIEH revealed the electrostatic potential distributions at interfaces of the charged particles used for development in electrophotography and should lead to dramatic improvements in electrophotography.
[Show abstract][Hide abstract] ABSTRACT: We have established a unique technique to fabricate three-dimensional (3D) well-defined transition-metal oxide epitaxial nanostructures. Fabrication of epitaxial spinel ferrite Fe2.2Zn0.8O4 (FZO) nanowall wires with a tunable width down to 20 nm was achieved. Cross-sectional transmission electron microscopy revealed the existence of an epitaxially matched lateral interface between the FZO nanowall wire and the side surface of 3D-MgO substrate. Magnetoresistance measurements showed ferromagnetic properties of the FZO nanowall wire at 300 K. The role of antiphase boundaries on the functionalities of the FZO nanoconfined wire is discussed.
[Show abstract][Hide abstract] ABSTRACT: The magnetic and crystallographic microstructures in Fe2.5Zn0.5O4 (FZO) wires fabricated using nano-imprint lithography, pulsed laser deposition and a molybdenum lift-off mask technique were studied by transmission electron microscopy (TEM). A process using a focused ion beam completely separated the FZO wires from the insulating MgO substrate, and accordingly allowed in-depth TEM studies of the domain structures. Observations using energy-filtered TEM demonstrated good crystallinity of the FZO wires. Both Lorentz microscopy and electron holography studies revealed unexpectedly small magnetic domains (∼100 nm or smaller) due to a significant interaction with antiphase boundaries. The role of antiphase boundaries on the functionalities observed in the constrained wires (e.g., nonlinear I–V characteristics and large magnetoresistance) is discussed on the basis of these microscopic observations.
[Show abstract][Hide abstract] ABSTRACT: Advanced split-illumination electron holography was developed by employing two biprisms in the illuminating system to split an electron wave into two coherent waves and two biprisms in the imaging system to overlap them. A focused image of an upper condenser-biprism filament was formed on the sample plane, and all other filaments were placed in its shadow. This developed system makes it possible to obtain precise reconstructed object waves without modulations due to Fresnel fringes, in addition to holograms of distant objects from reference waves.
[Show abstract][Hide abstract] ABSTRACT: Crystallographic and magnetic domains produced in a spinel-type compound MnV2O4, which exhibits a type of giant magnetostriction attributed to twin boundary motion, have been studied using transmission electron microscopy techniques. Although MnV2O4 undergoes a displacive cubic-to-tetragonal transformation upon cooling, it does not show a well-defined habit plane (i.e. the plane with a specific index that is favored for minimizing the transformation) due to the small elongation/contraction in the lattice. Electron holography demonstrates a considerable reduction in the magnetic signal by cooling the tetragonal phase to 40 K. Despite the elimination of micrometer-scale ferrimagnetic domains, weak magnetic contrast still remained, indicating small residual magnetic domains in particular portions, such as in the crosshatch of twinning pairs.
Journal of Alloys and Compounds 11/2013; 577:S731-S735. · 2.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The charging effects of microfibrils of sciatic nerve tissues due to electron irradiation are investigated using electron holography. The phenomenon that the charging effects are enhanced with an increase of electron intensity is visualized through direct observations of the electric potential distribution around the specimen. The electric potential at the surface of the specimen could be quantitatively evaluated by simulation, which takes into account the reference wave modulation due to the long-range electric field.
Microscopy and Microanalysis 08/2013; 19(S5). · 1.76 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The charging effect due to electron irradiation in an electron microscope has been studied so far with incident electrons. Here we report on a new specimen holder to control the charging effect by using electrons emitted from an irradiation port in the holder while maintaining a constant intensity of the incident electron beam. Details of the charging effect, such as electric field variation, are expected to be investigated by electron holography. The new specimen holder was developed by modifying a double-probe piezodriving specimen holder to introduce an electron irradiation port in one of its two arms. As a result, the new modified specimen holder consists of a piezodriving probe and an electron irradiation port, both of which can be controlled in three dimensions, using piezoelectric elements and micrometers. We demonstrate that variations in the charging effect for epoxy resin and surface contamination can be observed by electron holography.
Journal of electron microscopy 04/2013; · 1.63 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Understanding magnetic degradation in interface regions is important for materials science and technologies. An essential problem is the significant depression of ferromagnetism observed in an antiphase boundary (APB), which induces material functionalities/anomalies such as pinning of magnetic domain walls, etc. However, magnetization analysis on APB remains challenging due to the difficulty of collecting magnetic information from such a nanometer-scale interface region. We here use electron holography in order to obtain magnetic information from the narrow APB region in a Heusler alloy Ni50Mn25Al12.5Ga12.5. The magnetic flux density in APB (∼5.6 nm in breadth) was determined at 0.04 T, and was only 12% of the value for the APB-free matrix. This magnetic depression could be explained by atomic disordering in the APB region, ascertained by high-resolution atomic-column imaging. It is expected that our microscopic approach can be used to understand anomalous interface magnetism observed in various magnetic compounds and/or spin devices.
[Show abstract][Hide abstract] ABSTRACT: We investigate the effectiveness of utilizing a conductive probe for a transmission electron microscope (TEM) to suppress charging caused by electron irradiation. To do this, the electric field around a charged collagen fibril was visualized by electron holography and then quantitatively analyzed by computer simulations. The electric field changed noticeably when the conductive probe was moved near the specimen and charging was drastically suppressed when the conductive probe directly touched the charged specimen. The causes of the change in the electric field and suppression of charging are briefly discussed.
Journal of electron microscopy 03/2013; · 1.63 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The magnetic field generated by a magnetic recording head is evaluated using electron holography. A magnetic recording head, which is connected to an electric current source, is set on the specimen holder of a transmission electron microscope. Reconstructed phase images of the region around the magnetic pole show the change in the magnetic field distribution corresponding to the electric current applied to the coil of the head. A simulation of the magnetic field, which is conducted using the finite element method, reveals good agreement with the experimental observations.
Journal of electron microscopy 01/2013; · 1.63 Impact Factor