Takuro Nagai

Skyrmion flow near room temperature in an ultralow current density

The manipulation of spin textures with electric currents is an important challenge in the field of spintronics. Many attempts have been made to electrically drive magnetic domain walls in ferromagnets, yet the necessary current density remains quite high (~10(7) A cm(-2)). A recent neutron study combining Hall effect measurements has shown that an ultralow current density of J~10(2) A cm(-2) can trigger the rotational and translational motion of the skyrmion lattice in MnSi, a helimagnet, within a narrow temperature range. Raising the temperature range in which skyrmions are stable and reducing the current required to drive them are therefore desirable objectives. Here we demonstrate near-room-temperature motion of skyrmions driven by electrical currents in a microdevice composed of the helimagnet FeGe, by using in-situ Lorentz transmission electron microscopy. The rotational and translational motions of skyrmion crystal begin under critical current densities far below 100 A cm(-2).

Element-selective imaging of atomic columns in a crystal using STEM and EELS

Microstructure characterization has become indispensable to the study of complex materials, such as strongly correlated oxides, and can obtain useful information about the origin of their physical properties. Although atomically resolved measurements have long been possible, an important goal in microstructure characterization is to achieve element-selective imaging at atomic resolution. A combination of scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) is a promising technique for atomic-column analysis. However, two-dimensional analysis has not yet been performed owing to several difficulties, such as delocalization in inelastic scattering or instrumentation instabilities. Here we demonstrate atomic-column imaging of a crystal specimen using localized inelastic scattering and a stabilized scanning transmission electron microscope. The atomic columns of La, Mn and O in the layered manganite La1.2Sr1.8Mn2O7 are visualized as two-dimensional images.

Helical Carbon and Graphitic Films Prepared from Iodine-Doped Helical Polyacetylene Film Using Morphology-Retaining Carbonization

In this communication, we report a novel preparation of the helical carbon nanofibril-fabricated thin film from the iodine-doped filmy helical polyacetylene through a carbonization process. Carbonization of the helical polyacetylene films by way of iodine doping is found to afford carbon and graphitic films completely preserving morphologies and even helical nanofibril structures.

Local crystal structure analysis with several picometer precision using scanning transmission electron microscopy

We report a local crystal structure analysis with a high precision of several picometers on the basis of scanning transmission electron microscopy (STEM). Advanced annular dark-field (ADF) imaging has been demonstrated using software-based experimental and data-processing techniques, such as the improvement of signal-to-noise ratio, the reduction of image distortion, the quantification of experimental parameters (e.g., thickness and defocus) and the resolution enhancement by maximum-entropy deconvolution. The accuracy in the atom position measurement depends on the validity of the incoherent imaging approximation, in which an ADF image is described as the convolution between the incident probe profile and scattering objects. Although the qualitative interpretation of ADF image contrast is possible for a wide range of specimen thicknesses, the direct observation of a crystal structure with deep-sub-angstrom accuracy requires a thin specimen (e.g., 10nm), as well as observation of the structure image by conventional high-resolution transmission electron microscopy.

Molybdenum–Vanadium‐Based Molecular Sieves with Microchannels of Seven‐Membered Rings of Corner‐Sharing Metal Oxide Octahedra

Crystalline microporous oxides such as zeolites are indispensable materials in various applications ranging from industrial processes to everyday life, such as catalysts, ion-exchange materials, and molecular sieves. Most of them contain tetrahedrally coordinated metal atoms, but octahedrally coordinated metal centers have recently attracted much attention as building blocks of crystalline microporous metal oxides. Manganese oxides (pyrolusite, hollandite, todorokite, and romanechite) with micropores are the only crystalline porous materials based solely on octahedra (octahedral molecular sieves). These manganese oxides contain microtunnel pores consisting of {MnO6} octahedra that share edges and corners. Here we describe a novel type of octahedral molecular sieve, namely, crystalline orthorhombic Mo3VOx (x = 11.2), in which the microchannel is constructed by seven-membered rings of corner-sharing MO6 (M = Mo or V) octahedra. It is isostructural to orthorhombic MoVNbTeO compounds, which are very active and selective oxidation catalysts for light alkanes. These mixed metal oxides have a layered orthorhombic structure with a slab composed of sixand seven-membered rings of corner-sharing {MO6} octahedra and pentagonal {(M)M5O27} units with a {MO7} pentagonal bipyramid and five edge-sharing {MO6} octahedra, whereM is Mo, V, or Nb. The layered sixand seven-membered rings form channel structures. The Te atom is believed to be located both in the sixand seven-membered rings and block the channel. Recently, we succeeded in preparing an orthorhombic Mo3VOx compound that contains only Mo and V, [6] in which the channel is expected not to be blocked (Figure 1).

Non-nitrogen doped and non-metal oxygen reduction electrocatalysts based on carbon nanotubes: mechanism and origin of ORR activity

We show high activity in non-nitrogen doped and non-metal electrocatalysts based on carbon nanotubes with onset potential up to 0.73 V vs. RHE by the formation of hole defects on the walls of carbon nanotubes, followed by annealing under Ar atmosphere. From the power generation test, this catalyst can deliver a maximum output power of 55.68 mW (mg−1CNT cathode). Through temperature programmed desorption (TPD) and electrochemical analysis, the creation of new active sites is correlated with the removal of high temperature CO desorbing functionalities. Residual metal impurities were examined by the use of inductively coupled plasma-mass spectroscopy (ICP-MS), high angle annular dark-field (HAADF) and electron energy loss (EELS) analysis. The extremely low amount of metal impurities and the absence of impurity coordination at the edge planes after electrochemical characterization suggest that it is unlikely that impurities directly contribute to ORR. We conclude by proposing that the origin of ORR activity is a result of carbon restructuring and the possible formation of topological defects during the removal of high temperature CO desorbing functional groups.

Local crystal structure analysis with 10-pm accuracy using scanning transmission electron microscopy

We demonstrate local crystal structure analysis based on annular dark-field (ADF) imaging in scanning transmission electron microscopy (STEM). Using a stabilized STEM instrument and customized software, we first realize high accuracy of elemental discrimination and atom-position determination with a 10-pm-order accuracy, which can reveal major cation displacements associated with a variety of material properties, e.g. ferroelectricity and colossal magnetoresistivity. A-site ordered/disordered perovskite manganites Tb(0.5)Ba(0.5)MnO(3) are analysed; A-site ordering and a Mn-site displacement of 12 pm are detected in each specific atomic column. This method can be applied to practical and advanced materials, e.g. strongly correlated electron materials.

Entanglement-free fibrils of aligned polyacetylene films that produce single nanofibers

An aligned polyacetylene (PA) film was synthesized in a macroscopically-aligned nematic liquid crystal (N-LC) solvent using a gravity-flow method. Long and single nanofibers of less than 100 nm in radius were successfully prepared by ultrasonication of the aligned PA film immersed in ethanol. The usual PA film was synthesized in an isotropic solvent, such as toluene, only yielding short and non-dispersed fibers after the ultrasonication due to the entangled fibril morphology. Entanglement-free fibrils of the aligned PA were well-separated into single fibrils through ultrasonication, even without a surfactant.

Assessment of lower-voltage TEM performance using 3D Fourier transform of through-focus series

We assess the imaging performance of a transmission electron microscopy (TEM) system operated at a relatively low acceleration voltage using the three-dimensional (3D) Fourier transform of through-focus images. Although a single diffractogram and the Thon diagram cannot distinguish between the linear and non-linear TEM imaging terms, the 3D Fourier transform allows us to evaluate linear imaging terms, resulting in a conclusive assessment of TEM performance. Using this method, information transfer up to 98 pm is demonstrated for an 80 kV TEM system equipped with a spherical aberration corrector and a monochromator. We also revisit the Young fringe method in the light of the 3D Fourier transform, and have found a considerable amount of non-linear terms in Young fringes at 80 kV even from a typical standard specimen, such as an amorphous Ge thin film.

Variation of charge/orbital ordering in layered manganite Pr 1 − x Ca 1 + x Mn O 4 investigated by transmission electron microscopy

Structural features of the charge/orbital ordering