Shoichi Tomiyoshi

Magnetic Structure of Ca2Fe2O5

Magnetic measurements indicate that Ca 2 Fe 2 O 5 is an antiferromagnet, with weak parasitic ferromagnetism. The lattice, with the space group Pcmn -D 2 h 16 , can be considered as an oxygen deficient perovskite. The magnetic structure as determined from a neutron diffraction study is of G -type, and Fe 3+ spins are directed approximately along c -axis.

Temperature-Dependent Distribution of Internal Magnetic Fields at Fe57 Nuclei in fcc Iron-Nickel Alloys

Distribution of the internal magnetic fields and its temperature dependence of Fe 57 nuclei in fcc iron-nickel alloys with composition ranging from 31.8 to 65.6 at.% Ni have been measured by Mossbauer effect from 88°K to above Curie temperature. Distribution of the internal field has been determined from the observed Mossbauer spectra and temperature variation of the distribution has been explained by using the molecular field model which shows that iron moments on which weaker exchange field is acting from the neighbouring moments decrease more rapidly with increasing temperature than those on which stronger exchange field is acting. Distribution of the internal magnetic field has been correlated to the distribution of the exchange field. Temperature dependence of saturation magnetization and other magnetic properties of iron-nickel alloys has been discussed on the basis of local fluctuation of the exchange field.

High Resolution Neutron Powder Diffractometer with a Solid Methane Moderator at Pulsed Spallation Neutron Source

A high-resolution neutron powder diffractometer has been constructed at the pulsed spallation neutron facility KENS. The instrument utilizes a narrow pulsed beam of thermal and epithermal neutrons from a solid methane moderator. A high resolution of Δd/d~3×10-3 has been realized over a wide d range with a good data accumulation rate and with an excellent signal-to-background ratio. An off-line time-focussing method is introduced in order to obtain better statistics. A Rietveld refinement technique with a new profile shape function is briefly described. Some examples of measurements using a unique capability of this instrument are also presented.

Effect of Annealing on Thermoelectric Properties of Bismuth Telluride Compounds

The thermoelectric properties of the p-type (Bi0.25Sb0.75)2Te3 doped with 3 wt% excess Te and the n-type Bi2(Te0.94Se0.06)3 doped substantially with 0.07 wt% I and 0.02 wt% Te which were grown by the Bridgman method at a rate of 6 cm/h were measured before and after annealing, where the annealing was done in the temperature range from 473 up to 673 K for 1–10 h in vacuum and in a hydrogen stream. When annealing was done at 673 K for 2 h in vacuum, the power factor at 298 K for the p-type specimen increased by 35% from 4.25×10-3 to 5.72×10-3 W/mK2. In contrast, when annealing was done at 673 K for 5 h in a hydrogen stream, the power factor at 298 K for the n-type specimen increased by 24% from 4.82×10-3 to 5.99×10-3 K-1. The thermal conductivities κ at 298 K for both annealed specimens then decreased by about 5%. As a result, the maximum thermoelectric figures of merit Z at 298 K for the annealed p- and n-type specimens reached surprisingly large values of 4.16×10-3 K-1 and 3.99×10-3 K-1, respectively, with corresponding ZT values of 1.26 and 1.19. The reason that the Z values of bismuth telluride compounds were significantly enhanced by such an annealing is also discussed.

Magnetic Structure and Weak Ferromagnetism of Mn3Sn Studied by Polarized Neutron Diffraction

The magnetic structure and weak ferromagnetism of Mn 3 Sn were studied by polarized neutron diffraction at room temperature. The flipping ratio was measured for a number of Bragg reflections by rotating the crystal about the scattering vector under a magnetic field of 8 kOe applied perpendicular to the scattering vector. It was found that the spin structure has a triangular configuration of inverse geometry in the c -plane and the triangle rotates opposite to the rotation of the c -plane field component. From the analysis of the anisotropy energy it was shown that the triangle of inverse geometry is stabilized by the Dzyaloshinski-Moriya interaction and the weak ferromagnetic moment appears from the mechanism that each spin in the inverse triangle tilts slightly toward its easy axis. It was also shown that the weak ferromagnetic moment rotates just opposite to the rotation of the triangle.

Helical Spin Structure of Mn3Si

The magnetic structure of Mn 3 Si (bcc Fe 3 Al type crystal structure) at low temperatures has been determined by neutron diffraction using single crystal samples. It is found that Mn 3 Si has a helical spin structure (Neel temperature 25.8 K) with a wave vector q parallel to one of directions and of magnitude 0.85·1/2·2π K 111 , which is near the boundary of the Brillouin zone. The magnetic structure is either a proper screw or a transversal sinusoidal structure and the magnetic moments at the two Mn sites are µ Mn I =1.72 µ B and µ Mn II =0.19 µ B . The temperature dependence of the magnitude of the wave vector q is similar to that of Cr. From these data the possibility of an itinerant electron antiferromagnetism of Mn 3 Si is suggested in comparison with the SDW in Cr.

Neutron Diffraction Study of Cr2As

Cr 2 As is known as an antiferromagnet with the Neel temperature T N of 393 K. Its magnetic structure is determined by neutron diffraction by using single crystals as well as powdered samples. The magnetic structure obtained has such a symmetry that any isotropic exchange interaction vanishes between Cr(I) and Cr(II) sublattices, while dipole-dipole like interactions remain finite between them. Values of 0.40±0.08 µ B and 1.34±0.06 µ B are obtained for the magnetic moments of Cr(I) and Cr(II) respectively. Contrary to a previous study, it is observed that the magnetizations of Cr(I) and Cr(II) sublattices vanish at the same temperature, T N . This inconsistency is considered to come from the mis-indexing of lines which are actually due to Cr 2 O 3 contamination in the previous work.

Proton NMR in hydrogen-doped superconductor YBa2Cu3O7-δH0.2

Abstract Proton NMR in YBa 2 Cu 3 O 7-δ H 0.2 was performed between 77 K and 300 K. The hydrogen atom is diffusing or moving in the crystal above 170 K. Below 150 K it is trapped in the O(1) or O(2) oxygen sites in CuO chain. In the superconducting state below T c of 92 K, the penetration depth of the magnetic field at T = 0 K was determined to be 2500 A.

The Mössbauer Study of FeGe

The Mossbauer spectra of Fe 57 in FeGe were measured over the temperature range from 79°K to 413°K. The hyperfine field due to the antiferro-magnetic ordering was observed. Its value at 79°K was 155 kOe. The Neel point as determined from the temperature dependence of the internal magnetic field was 400°K. The isomer shift at room temperature was +0.22 mm/s. relative to pure Fe and varied with temperature like that of Fe. The electric quadrupole splitting \(\frac{1}{2}e^{2}qQ\) was +0.35 mm/sec at room temperature.

Preparation of Oriented Orthorhombic LnBa2Cu3O7-x Polycrystals

Tetragonal YBa2Cu3O7-x and DyBa2Cu3O7-x crystals were ground to resemble plate-like powder. Pellets of oriented tetragonal polycrystals were prepared from this powder using a uniaxial press. The tetragonal pellet was transformed into oriented orthorhombic polycrystals. The orientation of crystals was investigated by X-ray diffraction measurement and observed by SEM images. Samples exhibited zero resistance above 90 K, with a transition width of 2.0 K.