Unconventional supercurrent phase in Ising superconductor Josephson junction with atomically thin magnetic insulator.

In two-dimensional (2D) NbSe2 crystal, which lacks inversion symmetry, strong spin-orbit coupling aligns the spins of Cooper pairs to the orbital valleys, forming Ising Cooper pairs (ICPs). The unusual spin texture of ICPs can be further modulated by introducing magnetic exchange. Here, we report unconventional supercurrent phase in van der Waals heterostructure Josephson junctions (JJs) that couples NbSe2 ICPs across an atomically thin magnetic insulator (MI) Cr2Ge2Te6. By constructing a superconducting quantum interference device (SQUID), we measure the phase of the transferred Cooper pairs in the MI JJ. We demonstrate a doubly degenerate nontrivial JJ phase (ϕ), formed by momentum-conserving tunneling of ICPs across magnetic domains in the barrier. The doubly degenerate ground states in MI JJs provide a two-level quantum system that can be utilized as a new dissipationless component for superconducting quantum devices. Our work boosts the study of various superconducting states with spin-orbit coupling, opening up an avenue to designing new superconducting phase-controlled quantum electronic devices.

Spontaneous Polarization and Bulk Photovoltaic Effect Driven by Polar Discontinuity in LaFeO_{3}/SrTiO_{3} Heterojunctions.

Structurally coherent and chemically abrupt interfaces formed between polar and nonpolar perovskite oxides provide an ideal platform for examining the purely electronic reconstruction known as the polar catastrophe and the emergence of mobile or bound charges at the interface. The appearance of mobile charges induced by the polar catastrophe is already established in the LaAlO_{3}/SrTiO_{3} heterojunctions. Although not experimentally verified, the polar catastrophe can also lead to the emergence of spontaneous polarization. We report that thin films of originally nonpolar LaFeO_{3} grown on SrTiO_{3} are converted to polar as a consequence of the polar catastrophe. The induced spontaneous polarization evokes photovoltaic properties distinct from conventional p-n junctions, such as a switching of the photocurrent direction by changing the interfacial atomic sequence. The control of the bulk polarization by engineering the interface demonstrated here will expand the possibilities for designing and realizing new polar materials with photovoltaic functions.

Application of strain to orbital-spin-coupled system MnV2O4 at cryogenic temperatures within a transmission electron microscope.

The impact of mechanical stress on the morphology of crystallographic and magnetic domains in shape-controlled specimens of an orbital-spin-coupled system, MnV2O4, was examined by cryogenic Lorentz microscopy. Because of the difference in thermal expansion coefficients of MnV2O4 and the supporting Mo mesh, compression on the order of 0.01% was applied to the thin-foil specimens near the structural/magnetic phase transformation temperatures. The extent of compression was comparable to the lattice striction associated with the cubic-to-tetragonal phase transformation in MnV2O4 The applied strain thus clearly influenced the morphology of crystallographic domains (i.e. twinning configuration in the tetragonal phase) produced during cooling. The magnetic domain structure was entirely dependent on the configuration of twinning in the tetragonal phase. The observations in this study provided useful information for understanding the relationship between the crystallographic domains and the magnetic domains in MnV2O4.

Large anisotropic deformation of skyrmions in strained crystal.

Mechanical control of magnetism is an important and promising approach in spintronics. To date, strain control has mostly been demonstrated in ferromagnetic structures by exploiting a change in magnetocrystalline anisotropy. It would be desirable to achieve large strain effects on magnetic nanostructures. Here, using in situ Lorentz transmission electron microscopy, we demonstrate that anisotropic strain as small as 0.3% in a chiral magnet of FeGe induces very large deformations in magnetic skyrmions, as well as distortions of the skyrmion crystal lattice on the order of 20%. Skyrmions are stabilized by the Dzyaloshinskii-Moriya interaction, originating from a chiral crystal structure. Our results show that the change in the modulation of the strength of this interaction is amplified by two orders of magnitude with respect to changes in the crystal lattice due to an applied strain. Our findings may provide a mechanism to achieve strain control of topological magnetic structures based on the Dzyaloshinskii-Moriya interaction.

Magnetization amplified by structural disorder within nanometre-scale interface region.

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.

Ferromagnetic domain nucleation and growth in colossal magnetoresistive manganite.

Colossal magnetoresistance is a dramatic decrease in resistivity caused by applied magnetic fields, and has been the focus of much research because of its potential for magnetic data storage using materials such as manganites. Although extensive microscopy and theoretical studies have shown that colossal magnetoresistance involves competing insulating and ferromagnetic conductive phases, the mechanism underlying the effect remains unclear. Here, by directly observing magnetic domain walls and flux distributions using cryogenic Lorentz microscopy and electron holography, we demonstrate that an applied magnetic field assists nucleation and growth of an ordered ferromagnetic phase. These results provide new insights into the evolution dynamics of complex domain structures at the nanoscale, and help to explain anomalous phase separation phenomena that are relevant for applications. Our approach can also be used to determine magnetic parameters of nanoscale regions, such as magnetocrystalline anisotropy and exchange stiffness, without bulk magnetization results or neutron scattering data.

Magnetic resonance force microscopy combined with surface topography.

A new method of surface microscopy is proposed, which combines three-dimensional electron spin resonance imaging by magnetic resonance force microscopy (MRFM) and topographic imaging of the sample surface by scanning force microscopy (SFM). In order to demonstrate its potential for the identification of microscale objects, the individual and combined images are used to provide the locations, shapes and spin density distributions of target phantom objects. We report spatial resolution in MRFM of 2.8 x 2.8 x 2.0 microm(3). This could be improved to the theoretical limit of 0.08 x 0.08 x 0.04 microm(3) through reduction of the thermal noise by cooling to cryogenic temperatures approximately 0.5K. We believe that this type of microscopy will become a very useful tool for the investigation of anomalies induced in surfaces by materials buried below the surface.

Three dimensional magnetic resonance imaging by magnetic resonance force microscopy with a sharp magnetic needle.

An electropolished magnetic needle made of Nd(2)Fe(14)B permanent magnet was used for obtaining better spatial resolution than that achieved in our previous work. We observed the magnetic field gradient |G(Z)|=80.0G/microm and the field strength B=1250G at Z approximately 8.8 microm from the top of the needle. The use of this needle for three dimensional magnetic resonance force microscopy at room temperature allowed us to achieve the voxel resolution to be 0.6 microm x 0.6 microm x 0.7 microm in the reconstructed image of DPPH phantom. The acquisition time spent for the whole data collection over 64 x 64 x 16 points, including an iterative signal average by six times per point, was about 10 days.

Development of a magnetizing stage for in situ observations with electron holography and Lorentz microscopy.

A magnetizing stage, by which approximately horizontal magnetic fields can be applied to thin-foiled specimens, has been developed so that magnetization process can be observed in situ with electron holography and Lorentz microscopy. It is possible to apply magnetic field up to 200 Oe without serious image distortion by utilizing the magnetizing stage, beam-deflection-back coils and a magnetically shielded objective lens. The devised system can be used to studies of magnetization processes in many soft magnets.

Interaction of separated ferromagnetic domains in a hole-doped manganite achieved by a magnetic field.

We report the change in the magnetic microstructure with the application of a magnetic field to a hole-doped manganite La0.81Sr0.19MnO3 in the mixed-phase state, in which ferromagnetic and paramagnetic phases coexist. In situ observations by electron holography have revealed that the applied magnetic field generates a "channel" of the magnetic flux in the paramagnetic phase region, thereby connecting the separated ferromagnetic domains. The magnetic flux density of this channel is estimated at 0.33 T, which is comparable with that of the ferromagnetic domains. The connection of the separated ferromagnetic domains appears to promote the conduction in the mixed-phase state as predicted for many manganites exhibiting the magnetoresistance effect.

Magnetization distribution in the mixed-phase state of hole-doped manganites.

The effect of 'colossal magnetoresistance' (CMR) in hole-doped manganites--an abnormal decrease of resistivity when a magnetic field is applied--has attracted significant interest from researchers in the past decade. But the underlying mechanism for the CMR phenomenon is not yet fully understood. It has become clear that a phase-separated state, where magnetic and non-magnetic phases coexist, is important, but the detailed magnetic microstructure of this mixed-phase state is so far unclear. Here we use electron microscopy to study the magnetic microstructure and development of ferromagnetic domains in the mixed-phase state of La(1-x)Sr(x)MnO3 (x = 0.54, 0.56). Our measurements show that, in the absence of a magnetic field, the magnetic flux is closed within ferromagnetic regions, indicating a negligible magnetic interaction between separated ferromagnetic domains. However, we also find that the domains start to combine with only very small changes in temperature. We propose that the delicate nature of the magnetic microstructure in the mixed-phase state of hole-doped manganites is responsible for the CMR effect, in which significant conduction paths form between the ferromagnetic domains upon application of a magnetic field.

Precursor effects of martensitic transformations in Ti-based alloys studied by electron microscopy with energy filtering.

Precursor effects of martensitic transformations in two well-known shape memory alloys, Ti50Ni48Fe2 and Ti50Pd34Fe16, were studied extensively by energy-filtered electron microscopy, including in-situ observations, and high-resolution electron microscopy. Energy-filtered dark-field images, where weak diffuse scattering was utilized, clearly showed the microstructure in the premartensitic state of Ti50Ni48Fe2. Tiny domains observable in this state were close to spherical rather than thin and slender, and the temperature dependence of the domain-like structure was clarified by the in-situ observations. It was found that Ti50Pd34Fe16 exhibited a domain-like structure similar to that of Ti50Ni48Fe2, which was attributed to a transverse lattice displacement. High-resolution images of Ti50Pd34Fe16 showed that a domain was coupled with other ones with different orientations of distortion, so as to reduce the total strain due to their formations. Furthermore, effects of including a fundamental reflection, in addition to the diffuse scattering, on dark-field images were discussed based on the observations and the image processing.

New source of stacking faults in heteroepitaxial systems.

A new stacking fault formation mechanism has been observed for the first time in ZnO/LiTaO(3) heteroepitaxial films. High resolution electron microscopy studies combined with electron diffraction and numerical image computation suggest that the observed type I1 intrinsic stacking faults in an epitaxial film can be dominantly formed as a result of tilting of the lattices between films and substrate required to maintain a particular orientation relationship.

HRTEM image contrast of short range order in Ni4Mo

The high resolution transmission electron microscope (HRTEM) imaging of short range order (SRO) in Ni4Mo was investigated by means of multi-slice image simulations. The HRTEM images of Ni4Mo exhibit locally bright dot patterns corresponding to the [001] projections of the N2M2-type (chalcopyrite-like) structure. The multi-slice simulations revealed that the N2M2 patterns are rationalized as the projection patterns of the SRO structure which consists of subunit cell clusters of D1a, D022 and Pt2Mo structures. The N2M2-type image contrast appears when both the fundamental fcc lattice reflections and the 1 1/2 0 diffuse scattering of SRO contribute enough to imaging. This suggests that a good coincidence in intensity distribution between the Fourier power spectra of HRTEM images and the electron diffraction patterns is one of the conditions for the image contrast of SRO to be interpreted in terms of the projection contrast.

Computer Simulations for the Growth Process of Peanut-Type Hematite Particles.

The growth process of a monodispersed peanut-type hematite particle consisting of much smaller elongated subcrystals has been reproduced by computer simulation. On the basis of the microscopic internal structure, the simulation was performed by replacing the sequential events of surface nucleation and the subsequent growth of each subcrystal in the actual process with an arranged deposition of rectangular segments of fixed dimensions under different conditions in their site-dependent deposition rate and in the flexibility of their tilt angles. The most successful simulation model was obtained on the assumptions of a relatively fast deposition of the segments on the outermost side-surface of the ellipsoidal particle and the flexible tilt angle of each segment depending on the position of a neighboring new segment placed afterward. The result of the simulation strongly supported the previous elucidation of the growth mechanism in terms of the outward bending of adjoining subcrystals by nucleation and growth of a new subcrystal in each space between them. Enhancement of the outward bending of subcrystals by a large amount of sulfate ions adsorbed to the growing subcrystals was also suggested. Copyright 1999 Academic Press.

Internal Structure Analysis of Monodispersed Pseudocubic Hematite Particles by Electron Microscopy.

Morphology and internal structure of monodispersed pseudocubic hematite (alpha-Fe(2)O(3)) particles produced by the gel-sol method were investigated in detail through high-resolution electron microscopy on their thin sections prepared with an ultramicrotome. It was confirmed that the c-axis of a particle corresponded to the longest diagonal axis of the particle. High-resolution electron micrographs of thin sections directly revealed the polycrystallinity of the particles and clearly showed the arrangement of the subcrystals and their crystallographic orientations. The subcrystals near the surface of a particle were of a rectangular shape bound by the $ \{01\overline{1}2\} $ planes, and their width was about 12-16 nm with small variation of the length depending on the position in a particle. The subcrystals were radially developed from the center of a particle in all directions, but most preferentially in the longest diagonal axis of a particle. Besides, it was found from EDX analysis that the adsorbed Cl(-) ions as a shape controller were incorporated into the pseudocubic hematite particles during their growth.

High-resolution electron microscopy study on crystal structures of high-Tc superconductors.

Recent studies of high-resolution electron microscopy on the high-Tc superconductors of Y-Ba-Cu-O and Bi-Ca-Sr-Cu-O are presented. The observed images of crystals thinner than 3 nm, taken under conditions that approached the Scherzer defocus condition, directly show the arrangements of cations and oxygen-vacant positions. The results reveal structural characteristics of the atomic scale; this offers important insights into the origin of the high-Tc superconductivity. The usefulness of high-resolution electron microscopy for studying complicated crystal structures is demonstrated for the high-Tc oxides.