Hironari Okada

Observation of large magnetoresistance of magnetic Heusler alloy Ni50Mn36Sn14 in high magnetic fields

The magnetic and electrical properties on magnetic Heusler alloy Ni50Mn36Sn14 were studied in magnetic fields up to 18T in 4.2–270K temperature range. It was found that at the vicinity of 160K the resistivity jump of 46% is accompanied by the magnetic phase transition. Furthermore, the large magnetoresistance effect of 50% by the magnetic field induced magnetic phase transition was observed.The magnetic and electrical properties on magnetic Heusler alloy Ni50Mn36Sn14 were studied in magnetic fields up to 18T in 4.2–270K temperature range. It was found that at the vicinity of 160K the resistivity jump of 46% is accompanied by the magnetic phase transition. Furthermore, the large magnetoresistance effect of 50% by the magnetic field induced magnetic phase transition was observed.

Superconductivity under high pressure in LaFeAsO

Electrical resistivity measurements under high pressures up to 29 GPa were performed for oxypnictide compound LaFeAsO. We found a pressure-induced superconductivity in LaFeAsO. The maximum value of Tc is 21 K at ~12 GPa. The pressure dependence of the Tc is similar to those of LaFeAsO1-xFx series reported previously.

Pressure-Induced Superconductivity in Iron Pnictide Compound SrFe2As2

Electrical resistivity under high pressure have been measured on nominally pure SrFe2As2 up to 14 GPa. The resistivity drop appeared with increasing pressure, and we clearly observed zero resistivity. The maximum of superconducting transition temperature (Tc) is 38 K. The value is corresponding to the one of optimally doping AFe2As2 (A=Sr, Ba) system with K+ ions at the A2+ site.

Pressure-induced superconductor-insulator transition in the spinel compound CuRh2S4

We performed resistivity measurements in CuRh2S4 under quasihydrostatic pressure of up to 8.0 GPa, and found a pressure-induced superconductor-insulator transition. Initially, with increasing pressure, the superconducting transition temperature T(c) increases from 4.7 K at ambient pressure to 6.4 K at 4.0 GPa, but decreases at higher pressures. With further compression, superconductivity in CuRh2S4 disappears abruptly at a critical pressure P(SI) between 5.0 and 5.6 GPa, when it becomes an insulator.

High-pressure Studies on Superconducting iron-based LaFeAsO1-xFx , LaFePO and SrFe2As2

Electrical resistivity and magnetic susceptibility measurements under high pressure were performed on iron-based superconductor LaFePO and LaFeAsO 1- x F x system. A steep increase in superconducting transition temperature ( T c ) was observed for LaFePO and “optimum-doped” and “over-doped” LaFeAsO 1- x F x . Pressure-induced superconductivity was confirmed in undoped LaFeAsO and SrFe 2 As 2 by electrical resistivity measurements under high pressure. X-ray diffraction measurements were also performed under high pressure up to 10 GPa for LaFePO and LaFeAsO 1- x F x system, where the anisotropic decrease of the lattice constants was observed with applying pressure.

75As-NMR Studies on LaFeAsO1-xFx (x=0.14) under a Pressure of 3 GPa

75 As-nuclear magnetic resonance (NMR) on an iron (Fe)-based superconductor LaFeAsO 1- x F x ( x =0.14) was performed under a pressure of 3 GPa. Enhancement of the superconducting transition temperature ( T c ) was confirmed from the relaxation rate (1/ T 1 ); T c goes up to 40 K by applying pressure up to 3 GPa. 1/ T 1 T , which is temperature independent just above T c and gives a measure of the density of states (DOS) at the Fermi energy, enhances by applying pressure. These facts suggest that an increase of the DOS leads to the enhancement of T c . On the other hand, anomalous behavior of 1/ T 1 T observed at high temperatures is suppressed by applying pressure.

Enhancement of the critical temperature of the pnictide superconductor LaFeAsO1-xFx studied via75As NMR under pressure

Tatsuya Nakano, Naoki Fujiwara Kenichiro Tatsumi, Hironari Okada, Hiroki Takahashi, Yoichi Kamihara, Masahiro Hirano, and Hideo Hosono 1 Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-Nihonmatsu-cyo, Sakyo-ku, Kyoto 606-8501, Japan∗ 2 Department of Physics, College of Humanities and Sciences, Nihon University, Sakurajosui, Setagaya-ku, Tokyo 156-8550 TRiP, Japan Science and Technology Agency (JST), Sanban-cho bldg. 5, Sanban-cho, Chiyoda-ku, Tokyo 102-0075, Japan Materials and Structures Labolatory (MSL), Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226-8503, Japan 5 ERATO-SORST, Japan Science and Technology Agency (JST), Sanban-cho bldg. 5, Sanban-cho, Chiyoda-ku, Tokyo 102-0075, Japan and 6 Frontier Research Center (FRC), Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226-8503, Japan (Dated: September 1 2009)

Electrical conductivity of a bulk metallic glass composite

The authors report the electrical conductivity of a bulk metallic glass (BMG) based composite fabricated by warm extrusion of a mixture of gas-atomized glassy powders and ductile α-brass powders. The conductivity of the BMG composite can be well modeled by the percolation theory and the critical percolation threshold volume of the high-conductive brass phase was estimated to be about 10%. It was found that the short irregular brass fibers can dramatically reduce the resistivity of the BMG, leading to an improved material with both high strength and good conductivity for functional applications.

Magnetic properties of Heusler compounds Ru2CrGe and Ru2CrSn

We have succeeded in synthesizing the Heusler compounds Ru2CrGe and Ru2CrSn and performed magnetization measurements in high magnetic fields up to 18T in order to investigate the magnetic properties of Ru2CrGe and Ru2CrSn. We confirmed that the compounds have an ordered Heusler L21 structure. It was found that Ru2CrGe is an antiferromagnet with the Neel temperature TN=13K and Ru2CrSn shows a spin-glass-like behavior below 7K. The Heisenberg exchange constants determined on the basis of the experimental results for Ru2CrGe are different from the values of Mn-based Heusler compounds.

Triboelectricity evaluation of single toner particle by electron holography

Understanding electrification is particularly important in materials science since the use of charged particles, e.g., the electrophotographic printer with toner particles, is one of the most successful applications of electrification. However, the charge generation mechanism still remains unclear due to the lack of an appropriate method for evaluating individual fine particles. In this study, we describe an approach for determining the charge of a single toner particle that uses electron holography in combination with a shielding technique. Two long-standing problems in holographic studies—namely, perturbation of the reference electron wave and unwanted charging by illumination—have been overcome by introducing two types of shields in a microscope. Using this method, the amount of charge on a single toner particle was determined, and the surface charge distribution was found to be inhomogeneous. Furthermore, an in situ observation of triboelectricity was conducted inside the microscope.