Electron holography observation of individual ferrimagnetic lattice planes.

Atomic-scale observations of a specific local area would be considerably beneficial when exploring new fundamental materials and devices. The development of hardware-type aberration correction1,2 in electron microscopy has enabled local structural observations with atomic resolution3-5 as well as chemical and vibration analysis6-8. In magnetic imaging, however, atomic-level spin configurations are analysed by electron energy-loss spectroscopy by placing samples in strong magnetic fields9-11, which destroy the nature of the magnetic ordering in the samples. Although magnetic-field-free observations can visualize the intrinsic magnetic fields of an antiferromagnet by unit-cell averaging12, directly observing the magnetic field of an individual atomic layer of a non-uniform structure is challenging. Here we report that the magnetic fields of an individual lattice plane inside materials with a non-uniform structure can be observed under magnetic-field-free conditions by electron holography with a hardware-type aberration corrector assisted by post-digital aberration correction. The magnetic phases of the net magnetic moments of (111) lattice planes formed by opposite spin orderings between Fe3+ and Mo5+ in a ferrimagnetic double-perovskite oxide (Ba2FeMoO6) were successfully observed. This result opens the door to direct observations of the magnetic lattice in local areas, such as interfaces and grain boundaries, in many materials and devices.

Current-Driven Motion of Domain Boundaries between Skyrmion Lattice and Helical Magnetic Structure.

To utilize magnetic skyrmions, nanoscale vortex-like magnetic structures, experimental elucidation of their dynamics against current application in various circumstances such as in confined structure and mixture of different magnetic phases is indispensable. Here, we investigate the current-induced dynamics of the coexistence state of magnetic skyrmions and helical magnetic structure in a thin plate of B20-type helimagnet FeGe in terms of in situ real-space observation using Lorentz transmission electron microscopy. Current pulses with various heights and widths were applied, and the change of the magnetic domain distribution was analyzed using a machine-learning technique. The observed average driving direction of the two-magnetic-state domain boundary is opposite to the applied electric current, indicating ferromagnetic s-d exchange coupling in the spin-transfer torque mechanism. The evaluated driving distance tends to increase with increasing the pulse duration time, current density (>1 × 109 A/m2), and sample temperature, providing valuable information about hitherto unknown current-induced dynamics of the skyrmion-lattice ensemble.

Three-dimensional observation of magnetic vortex cores in stacked ferromagnetic discs.

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 to 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.

Real-Space Observation of Short-Period Cubic Lattice of Skyrmions in MnGe.

Three-dimensional forms of skyrmion aggregate, such as a cubic lattice of skyrmions, are anticipated to exist, yet their direct observations remain elusive. Here, we report real-space observations of spin configurations of the skyrmion-antiskyrmion cubic-lattice in MnGe with a very short period (∼3 nm) and hence endowed with the largest skyrmion number density. The skyrmion lattices parallel to the {100} atomic lattices are directly observed using high-resolution Lorentz transmission electron microscopes, simultaneously with underlying atomic-lattice fringes.

Electron holographic visualization of collective motion of electrons through electric field variation.

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.

Removal of phase residues in electron holography.

Electron holography provides quantitative phase information regarding the electromagnetic fields and the morphology of micro- to nano-scale samples. A phase image reconstructed numerically from an electron hologram sometimes includes phase residues, i.e. origins of unremovable phase discontinuities, which make it much more difficult to quantitatively analyze local phase values. We developed a method to remove the residues in a phase image by a combination of patching local areas of a hologram and denoising based on machine learning. The small patches for a hologram, which were generated using the spatial frequency information of the own fringe patterns, were pasted at each residue point by an algorithm based on sparse modeling. After successive phase reconstruction, the phase components with no dependency on the vicinity were filtered out by Gaussian process regression. We determined that the phase discontinuities that appeared around phase residues were removed and the phase distributions of an atomic resolution phase image of a Pt nanoparticle were sufficiently restored.

High-precision charge analysis in a catalytic nanoparticle by electron holography.

The charge state of supported metal catalysts is the key to understand the elementary processes involved in catalytic reactions. However, high-precision charge analysis of the metal catalysts at the atomic level is experimentally challenging. To address this critical challenge, high-sensitivity electron holography has recently been successfully applied for precisely measuring the elementary charges on individual platinum nanoparticles supported on a titanium dioxide surface. In this review, we introduce the latest advancements in high-precision charge analysis and discuss the mechanisms of charge transfer at the metal-support interface. The development of charge measurements is entering a new era, and charge analyses under conditions closer to practical working environments, such as real-time, real-space, and reactive gas environments, are expected to be realized in the near future.

Spontaneous Vortex-Antivortex Pairs and Their Topological Transitions in a Chiral-Lattice Magnet.

The spontaneous formation and topological transitions of vortex-antivortex pairs have implications for a broad range of emergent phenomena, for example, from superconductivity to quantum computing. Unlike magnets exhibiting collinear spin textures, helimagnets with noncollinear spin textures provide unique opportunities to manipulate topological forms such as (anti)merons and (anti)skyrmions. However, it is challenging to achieve multiple topological states and their interconversion in a single helimagnet due to the topological protection for each state. Here, the on-demand creation of multiple topological states in a helimagnet Fe0.5 Co0.5 Ge, including a spontaneous vortex pair of meron with topological charge N = -1/2 and antimeron with N = 1/2, and a vortex-antivortex bundle, that is, a bimeron (meron pair) with N = -1 is reported. The mutual transformation between skyrmions and bimerons with respect to the competitive effects of magnetic field and magnetic shape anisotropy is demonstrated. It is shown that electric currents drive the individual bimerons to form their connecting assembly and then into a skyrmion lattice. These findings signify the feasibility of designing topological states and offer new insights into the manipulation of noncollinear spin textures for potential applications in various fields.

Bloch Point Quadrupole Constituting Hybrid Topological Strings Revealed with Electron Holographic Vector Field Tomography.

Topological magnetic (anti)skyrmions are robust string-like objects heralded as potential components in next-generation topological spintronics devices due to their low-energy manipulability via stimuli such as magnetic fields, heat, and electric/thermal current. While these 2D topological objects are widely studied, intrinsically 3D electron-spin real-space topology remains less explored despite its prevalence in bulky magnets. 2D-imaging studies reveal peculiar vortex-like contrast in the core regions of spin textures present in antiskyrmion-hosting thin plate magnets with S4 crystal symmetry, suggesting a more complex 3D real-space structure than the 2D model suggests. Here, holographic vector field electron tomography captures the 3D structure of antiskyrmions in a single-crystal, precision-doped (Fe0.63Ni0.3Pd0.07)3P (FNPP) lamellae at room temperature and zero field. These measurements reveal hybrid string-like solitons composed of skyrmions with topological number W = -1 on the lamellae's surfaces and an antiskyrmion (W = + 1) connecting them. High-resolution images uncover a Bloch point quadrupole (four magnetic (anti)monopoles that are undetectable in 2D imaging) which enables the observed lengthwise topological transitions. Numerical calculations corroborate the stability of hybrid strings over their conventional (anti)skyrmion counterparts. Hybrid strings result in topological tuning, a tunable topological Hall effect, and the suppression of skyrmion Hall motion, disrupting existing paradigms within spintronics.

Nonmagnetic framboid and associated iron nanoparticles with a space-weathered feature from asteroid Ryugu.

Extraterrestrial minerals on the surface of airless Solar System bodies undergo gradual alteration processes known as space weathering over long periods of time. The signatures of space weathering help us understand the phenomena occurring in the Solar System. However, meteorites rarely retain the signatures, making it impossible to study the space weathering processes precisely. Here, we examine samples retrieved from the asteroid Ryugu by the Hayabusa2 spacecraft and discover the presence of nonmagnetic framboids through electron holography measurements that can visualize magnetic flux. Magnetite particles, which normally provide a record of the nebular magnetic field, have lost their magnetic properties by reduction via a high-velocity (>5 km s-1) impact of a micrometeoroid with a diameter ranging from 2 to 20 µm after destruction of the parent body of Ryugu. Around these particles, thousands of metallic-iron nanoparticles with a vortex magnetic domain structure, which could have recorded a magnetic field in the impact event, are found. Through measuring the remanent magnetization of the iron nanoparticles, future studies are expected to elucidate the nature of the nebular/interplanetary magnetic fields after the termination of aqueous alteration in an asteroid.

Spatial and phase resolution in electron microscopy.

With the invention of the aberration corrector in electron optics, the spatial resolution in electron microscopy has progressively improved and has now reached the sub-50-pm regime, and atomic-resolution electron microscopy has become a versatile tool for investigating the atomic structures in materials and devices. Furthermore, the phase resolution in electron microscopy also exhibits outstanding progress, and it has become possible to visualize electromagnetic fields at atomic dimensions, which strongly contributes to understanding the physical and chemical properties of materials. The electron microscopy society has grown with the improvements in spatial and phase resolutions, and hence, we must continuously develop new hardware, software and methodologies to boost these resolutions. Here, we review the historical progress of spatial and phase resolutions in electron microscopy, where we clarify the definition of these resolutions. We also discuss the future targets in electron microscopy.

Visualization of nanoscale magnetic domain states in the asteroid Ryugu.

In the samples collected from the asteroid Ryugu, magnetite displays natural remanent magnetization due to nebular magnetic field, whereas contemporaneously grown iron sulfide does not display stable remanent magnetization. To clarify this counterintuitive feature, we observed their nanoscale magnetic domain structures using electron holography and found that framboidal magnetites have an external magnetic field of 300 A m-1, similar to the bulk value, and its magnetic stability was enhanced by interactions with neighboring magnetites, permitting a disk magnetic field to be recorded. Micrometer-sized pyrrhotite showed a multidomain magnetic structure that was unable to retain natural remanent magnetization over a long time due to short relaxation time of magnetic-domain-wall movement, whereas submicron-sized sulfides formed a nonmagnetic phase. These results show that both magnetite and sulfide could have formed simultaneously during the aqueous alteration in the parent body of the asteroid Ryugu.

Three-dimensional reconstructions of electrostatic potential distributions with 1.5-nm resolution using off-axis electron holography.

Three-dimensional (3D) reconstruction experiments were carried out by observing high-resolution 3D electrostatic potential distributions of Pt nanoparticles using off-axis electron holographic tomography. These Pt nanoparticles were mounted on the surfaces of amorphous silicon pillars. In order to realize high-resolution observation, we developed a mechanically stable 3D specimen holder with small specimen drifts and vibrations. From the 3D electrostatic potential distribution data of Pt nanoparticles (2.0 nm in diameter), we obtained the resolution of 1.5 nm.

Magnetization distribution of magnetic vortex of amorphous FeSiB investigated by electron holography and computer simulation.

The three-dimensional spin structure of the magnetic vortex of FeSiB, an amorphous soft magnetic material, was investigated by holography observation and computer simulation. Magnetization distribution in the neighborhood of the vortex center was estimated from the phase distribution obtained by holography observation. To confirm this magnetization distribution, sample-tilting experiments were performed: when the sample was tilted with respect to the electron beam direction, the phase-image center was found to shift along the tilting axis. Finite-element computer simulation was carried out to estimate the amount of shifts of the phase-image center in the sample tilting from the experimental magnetization distributions in the no sample-tilting conditions. We found that the simulated shifts of the phase-image center were in good agreement with those in the sample-tilting experiment, thus confirming the magnetization distribution near the vortex center obtained by holography observation.

Direct identification of the charge state in a single platinum nanoparticle on titanium oxide.

A goal in the characterization of supported metal catalysts is to achieve particle-by-particle analysis of the charge state strongly correlated with the catalytic activity. Here, we demonstrate the direct identification of the charge state of individual platinum nanoparticles (NPs) supported on titanium dioxide using ultrahigh sensitivity and precision electron holography. Sophisticated phase-shift analysis for the part of the NPs protruding into the vacuum visualized slight potential changes around individual platinum NPs. The analysis revealed the number (only one to six electrons) and sense (positive or negative) of the charge per platinum NP. The underlying mechanism of platinum charging is explained by the work function differences between platinum and titanium dioxide (depending on the orientation relationship and lattice distortion) and by first-principles calculations in terms of the charge transfer processes.

Three-dimensional evaluation of an independent multi-walled carbon nanotube probe by tomography with high-resolution transmission electron microscope.

We evaluated an independent multi-walled carbon nanotube (MWNT) probe by using tomography with a high-resolution transmission electron microscope to verify the three-dimensional structure of the probe tip. The new method of probe evaluation revealed the following features: (i) cutting the end of the MWNT probe caused the wall structure to disintegrate and encapsulated the graphene sheets fragmented by the discharged pulse; (ii) the cap of the MWNT probe was an open cylinder covered by walls similar in shape to a rectangular slit; (iii) the grooves of the inner walls of the MWNT probe, which were created by the discharge cutting method, maintained a cylindrical shape that was different from the peeling-off mechanism.

Holography: application to high-resolution imaging.

Electron holography was invented for correcting aberrations of the lenses of electron microscopes. It was used to observe the atomic arrangements in crystals after decades of research. Then it was combined with a hardware aberration corrector to enable high-resolution and high-precision analysis. Its applications were further extended to magnetic observations with sub-nanometer resolution. High-resolution electron holography has become a powerful technique for observing electromagnetic distributions in functional materials.

Improved efficiency in automated acquisition of ultra-high-resolution electron holograms using automated target detection.

An automated hologram acquisition system for big-data analysis and for improving the statistical precision of phase analysis has been upgraded with automated particle detection technology. The coordinates of objects in low-magnification images are automatically detected using zero-mean normalized cross-correlation with preselected reference images. In contrast with the conventional scanning acquisitions from the whole area of a microgrid and/or a thin specimen, the new method allows efficient data collections only from the desired fields of view including the particles. The acquisition time of the cubic/triangular nanoparticles that were observed was shortened by about one-fifty eighth that of the conventional scanning acquisition method because of efficient data collections. The developed technology can improve statistical precision in electron holography with shorter acquisition time and is applicable to the analysis of electromagnetic fields for various kinds of nanoparticles.

Deep convolutional neural network image processing method providing improved signal-to-noise ratios in electron holography.

An image identification method was developed with the aid of a deep convolutional neural network (CNN) and applied to the analysis of inorganic particles using electron holography. Despite significant variation in the shapes of α-Fe2O3 particles that were observed by transmission electron microscopy, this CNN-based method could be used to identify isolated, spindle-shaped particles that were distinct from other particles that had undergone pairing and/or agglomeration. The averaging of images of these isolated particles provided a significant improvement in the phase analysis precision of the electron holography observations. This method is expected to be helpful in the analysis of weak electromagnetic fields generated by nanoparticles showing only small phase shifts.

Automated acquisition of vast numbers of electron holograms with atomic-scale phase information.

An automated acquisition system for collecting a large number of electron holograms, to improve the statistical precision of phase analysis, was developed. A technique for shifting the electron beam in combination with stage movement allows data to be acquired over a wide area of a TEM-specimen grid. Undesired drift in the hologram position, which may occur during the hologram acquisition, can be corrected in real time by automated detection of the interference-fringe region in an image. To demonstrate the usefulness of the developed automated hologram acquisition system, gold nanoparticles dispersed on a carbon foil were observed with a 1.2-MV atomic resolution holography electron microscope. The system could obtain 1024 holograms, which provided phase maps for more than 500 nanoparticles with a lateral resolution of 0.14 nm, in just 1 h. The observation results revealed an anomalous increase in mean inner potential for a particle size smaller than 4 nm. The developed automated hologram acquisition system can be applied to improve the precision of phase measurement by averaging many phase images, as demonstrated by single particle analysis for biological entities. Moreover, the system makes it possible to study electrostatic potential of catalysts and other functional nanoparticles at atomic resolution.