Kodai Niitsu

Stress-induced transformation behaviors at low temperatures in Ti-51.8Ni (at. %) shape memory alloy

Martensitic transformation behaviors at low temperatures in polycrystalline Ti-51.8Ni (at. %) alloy were investigated with compressive stress. Superelasticity with almost complete shape recovery was confirmed at temperatures of 40–180u2009K. In stress-induced transformation at fixed temperatures, while the reverse transformation finishing stress monotonically decreased with decreasing test temperature, the forward transformation starting stress changed from decrease to increase at about 125u2009K. Forward and reverse transformation temperatures evaluated under constant stresses of 50 and 750u2009MPa were generally coincident with the results determined at the constant temperatures, where “heating-induced forward transformation” was confirmed under a stress of 750u2009MPa.

Stress-induced transformations at low temperatures in a Ni45Co5Mn36In14 metamagnetic shape memory alloy

The martensitic transformation behavior in a Ni45Co5Mn36In14 was investigated at low temperatures. Almost perfect superelasticity was confirmed below 200u2009K. The reverse transformation finishing stress monotonically decreased with decreasing temperature and the forward transformation starting stress changed from a decrease to an increase at ∼125u2009K. The temperature dependence of equilibrium stress had the same tendency as that of equilibrium magnetic field, allowing the thermal transformation arrest temperature to be determined. The temperature dependence of hysteresis in stress-induced transformation was also similar to that in magnetic-field-induced transformation, and the levels of dissipation energy yielded by the stress and magnetic field were intrinsically equivalent.

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 (2–3u2009nm), within which significant atomic disordering occurs. Electron holography studies revealed an unexpectedly large magnetic flux density at the APBs, amplified by approximately 60% (at 293u2009K) 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.

Composition Dependences of Entropy Change and Transformation Temperatures in Ni-rich Ti–Ni System

For Ni-rich Ti–Ni alloys, physical properties such as specific heat and electric resistance were systematically investigated. The B2/B19′ martensitic transformation temperatures ranging from 180 to 373xa0K were determined for Ni contents of 49.98–51.09xa0%, and a sudden disappearance of martensitic transformation was confirmed for Ni contents greater than 51.23xa0%, which has also been well reported in the literatures. The entropy change was also evaluated from differential scanning calorimeter measurement, and it was clarified that the entropy change plotted to T0 temperature shows an S-shaped curve, starting to drastically decrease at about 300xa0K. Thermodynamic approaches were then carried out attempting to determine the reason for the disappearance of transformation. The entropy change estimated from direct measurements of specific heats for 51.75 Ni (B2) and 50.92 Ni (B19′) was found to be more consistent with the experimental data, rather than the calculated curve based on the Debye model for vibration specific heat. It was proposed that the equilibrium between the parent and martensite phases obeys the Clausius–Clapeyron relationship in the composition–temperature system. Using the constructed composition–temperature diagram, the disappearance of martensitic transformation in the Ti–Ni system can be well understood as being due to the drastic increase of hysteresis at low temperature.

Effect of Microalloying Elements on Solidification Microstructure of the La(Fe0.89Si0.11)13 Alloy

The Effect of Microalloying Elements and Compounds, such as Be, B, C, P, S, Ti, V, Cu, Zn, in, BN, VN, Mn3N, LaN, MnS and Ti4C2S2, Ranging in Amount from 0.005 to 0.2 at.%, on the α-Fe + FeLaSi Two-Phase Microstructure of a La(Fe0.89Si0.11)13 as-Melted Specimen Was Investigated. The Addition of Mn3N Was Found to Contribute to α-Fe Grain Refinement to a Certain Extent, but to Harm the Uniform Growth of the τ1 Phase in the Stage of Subsequent Annealing.

Magnetic field observations in CoFeB/Ta layers with 0.67-nm resolution by electron holography

Nanometre-scale magnetic field distributions in materials such as those at oxide interfaces, in thin layers of spintronics devices, and at boundaries in magnets have become important research targets in materials science and applied physics. Electron holography has advantages in nanometric magnetic field observations, and the realization of aberration correctors has improved its spatial resolution. Here we show the subnanometre magnetic field observations inside a sample at 0.67-nm resolution achieved by an aberration-corrected 1.2-MV holography electron microscope with a pulse magnetization system. A magnetization reduction due to intermixing in a CoFeB/Ta multilayer is analyzed by observing magnetic field and electrostatic potential distributions simultaneously. Our results demonstrate that high-voltage electron holography can be widely applied to pin-point magnetization analysis with structural and composition information in physics, chemistry, and materials science.

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.

Measurement of Vortex Beam Phase by Electron Holography

Electron vortex beams are considered as new probes for next-generation electron beam machines, especially for transmission electron microscopes, because the vortex beams carry intrinsic orbital angular momentum, leading to unprecedented measurement capabilities [1, 2]. To realize a practical measurement system with vortex beams, a few experiments were attempted to measure intensity distributions or energy-loss distributions [3]. In previous studies, we succeeded in controlling vortex beams in the reciprocal space by using special opening shapes of the fork-shaped gratings [4]. In the present study, electron holography techniques were used in the optical system to measure phases of vortex beams. The principle idea is the same as that of Gabor’s original in-line holography. Electron beams passed around the fork-shaped grating were used as the reference waves and defocused images from the diffraction pattern were recorded as holograms of the vortex beams.

Interference experiment with asymmetric double slit by using 1.2-MV field emission transmission electron microscope

Advanced electron microscopy technologies have made it possible to perform precise double-slit interference experiments. We used a 1.2-MV field emission electron microscope providing coherent electron waves and a direct detection camera system enabling single-electron detections at a sub-second exposure time. We developed a method to perform the interference experiment by using an asymmetric double-slit fabricated by a focused ion beam instrument and by operating the microscope under a “pre-Fraunhofer” condition, different from the Fraunhofer condition of conventional double-slit experiments. Here, pre-Fraunhofer condition means that each single-slit observation was performed under the Fraunhofer condition, while the double-slit observations were performed under the Fresnel condition. The interference experiments with each single slit and with the asymmetric double slit were carried out under two different electron dose conditions: high-dose for calculation of electron probability distribution and low-dose for each single electron distribution. Finally, we exemplified the distribution of single electrons by color-coding according to the above three types of experiments as a composite image.

Sub-Nanometer-Resolution Magnetic Field Observation Using Aberration-Corrected 1.2-MV Holography Electron Microscope with Pulse Magnetization System

1. Research & Development Group, Hitachi, Ltd., Hatoyama, Japan 2. RIKEN Center for Emergent Matter Science (CEMS), Wako, Japan 3. Hitachi High-Technologies Corporation, Hitachinaka, Japan 4. National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan 5. Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Japan 6. Department of Applied Physics and Quantum-Phase Electron Center (QPEC), University of Tokyo, Tokyo, Japan