Yoshikazu Makihara

Magnetic Characteristics of Laves Phase RMn2 Compounds (R=Gd, Tb, Dy, Ho and Er)

Measurements of magnetization, susceptibility, thermal expansion and electrical resistivity have been carried out on the RMn 2 compounds with R=Gd, Tb, Dy, Ho and Er. Magnetization measurements were made on the single crystals for all the compounds. A simple ferrimagnetic structure is observed at 4.2 K for GdMn 2 , HoMn 2 and ErMn 2 , and an antiferromagnetic one in TbMn 2 . While, a canted ferrimagnetic structure appears at 4.2 K in DyMn 2 . The Mn moment deduced from saturation magnetization decreases rapidly from 1.6 µ B for GdMn 2 to 0.2 µ B for HoMn 2 with increasing the atomic number of rare earth in the C15 compounds. While, the Mn moment is almost zero in ErMn 2 with the C14 structure. Anomalous discontinuities in the thermal expansion and resistivity for GdMn 2 and TbMn 2 are observed at the magnetic transition temperature where Mn moments lose a long range ordering. It seems likely that the discontinuity is associated with the onset of the Mn moment.

XAFS Study on Nitrogenation of CoTiSn

Gas-phase nitrogenation of CoTiSn compound with the MgAgAs-type structure has been examined by means of x-ray diffraction (XRD) and x-ray absorption fine structure (XAFS). The XRD suggests that the nitrogenation product is composed of the corresponding Cu2MnAl-type compound with expanded unit-cell volume comparable to that of Co2TiSn and subtle trace of Ti6Sn5 and Sn. Whereas the x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) spectra for the Ti, Co and Sn K-edges clearly indicate the coexistence of TiN and Co1+yTiSn phases in the product. It is also revealed that more Co atoms than expected from the chemical formulas are introduced into the vacancy site of the MgAgAs structure in Co1+yTiSn with 0≤y≤0.4.

Neutron diffraction study on the Heusler compound Co1.50TiSn and its nitrogenation products

Abstract Neutron powder diffraction studies have been performed for the Heusler compound ‘CoTiSn’ prepared from stoichiometric amounts of the constituent elements and its nitrogenation products. The Rietveld refinement indicates that the ‘CoTiSn’ sample consists of three phases: Co+Ti+Sn→0.69Co 1.50 TiSn+0.05Ti 6 Sn 5 +0.06Sn. It is concluded that the stoichiometric compound CoTiSn is not synthesized but that Co 1.50 TiSn with a half-filled vacancy sublattice in the MgAgAs-type structure which has the XYZ configuration is formed. Nitrogen absorption is accompanied by the disproportionation of the ‘CoTiSn’ sample according to the following equation: ‘CoTiSn’+0.23N 2 →0.50Co 1.92 TiSn+0.47TiN+0.36Sn+0.04CoSn 2 +0.03Co 1.50 TiSn. It is revealed that nitrogen atoms react selectively with Ti atoms in the Co 1.50 TiSn and Ti 6 Sn 5 compounds and that nitrogen-induced occupation of the vacancy site in Co 1.50 TiSn by Co atoms takes place during nitrogenation; Co 1.92 TiSn with the Cu 2 MnAl-type structure is formed.

Pressure effect on the magnetic properties of Ce(Fe1-xCox)2

We have studied the magnetic properties of Ce(Fe 1- x Co x ) 2 with x ≤0.1 by magnetization measurements under high pressures up to 1.2 GPa. In pure CeFe 2 , the Curie temperature T C decreases, and the saturation magnetization M s is suppressed by applying pressure. In Co substitution compounds, the transition temperature T A from the ferromagnetic (FM) to the antiferromagnetic (AFM) states and the metamagnetic transition field B c increase with increasing pressure, while T C and the magnetization of the spontaneous and field-induced FM states decrease. The obtained results indicate that the AFM state becomes stable by the lattice contraction.

X-Ray Diffraction Study on the Pseudobinary Compound Ce(Fe0.9Co0.1)2 in High Magnetic Fields

We have studied the magnetic and structural properties of Ce(Fe 0.9 Co 0.1 ) 2 with the C15 cubic Laves-phase structure by low-temperature x-ray diffraction measurements in magnetic fields up to 5 T, and in the temperature range from 9 to 300 K. The C15 structure is distorted to rhombohedral symmetry at 90 K in zero magnetic field, accompanied with the magnetic phase transition from the ferromagnetic (FM) to the antiferromagnetic (AFM) states with decreasing temperature. In the AFM state, it is observed clearly that the rhombohedral distortion state recovers the C15 structure by applying magnetic field, accompanied with the metamagnetic transition from the AFM to the forced FM states with a hysteresis in the magnetization process.

Magneto-volume effect in the ground and field-induced states of Ce2Fe17

Abstract X-ray diffraction measurements on Ce 2 Fe 17 have been performed in the temperature range from 8 to 300 K at the magnetic fields up to 5 T . The thermal expansion curves along the principal axes for antiferromagnet and ferromagnet Ce 2 Fe 17 are presented. A distinct lattice expansion was observed simultaneously with the metamagnetic transition. This suggests that the hybridization between Fe 3d- and Ce 4f-electrons in Ce 2 Fe 17 is suppressed by the application of magnetic field.

Structural and Hydriding/Dehydriding Properties of HoMn 2 H x with Cubic and Hexagonal Laves Phase Structure

Both the cubic C15-type and the hexagonal C14-type Laves phase compounds HoMn 2 were successfully prepared. We found that they absorbed hydrogen up to 3.4 atoms per formula unit at room temperature and discharged it at around 490 K, reversibly. Structural and hydriding/dehydriding properties of the host HoMn 2 and their hydrides HoMn 2 H x have been investigated by powder X-ray diffraction (XRD) and thermogravimetry/differential thermal analysis (TG/DTA). With increasing the hydrogen content, the host C15-type HoMn 2 is distorted into a rhombohedral phase at above x = 2.9, while the host C14-type one keeps the hexagonal structure up to x = 3.4. The experimental results obtained from TG/DTA measurement suggest that two possible interstitial sites with different activation energies for hydrogen desorption exist in the C14 HoMn 2 , while one site in the C15 HoMn 2 .

A new Heusler-type nitride TiCoNxSn produced by gas-phase nitrogenation

Abstract The result of gas-phase nitrogenation for the TiCoSn compound is reported. The cell volume of the synthesized nitride TiCoN 0.33 Sn with Heusler-type structure is 3.3% greater than for the parent compound. It is concluded that the TiCoSn nitride is a simple gas-solid solution with intermediate nitrogen contents and that the nitrogen atoms in the nitrides are introduced into the vacancy sublattice sites in the parent compound.

The drastic enhancement of ferromagnetic properties of a new Heusler-type nitride TiCoNxSn

Abstract Magnetic properties of a new class of itinerant Heusler-type nitrides TiCoN x Sn with 0 ⩽ x ⩽ 0.33 are reported. It is found that the Curie temperature and the saturation magnetization of the nitrides are drastically enhanced from 149 K and 0.39 μ B /Co to 362 K and 0.91 μ B /Co, almost to those of TiCo 2 Sn by introducing nitrogen atoms into the vacancy sublattice sites of MgAgAs-type TiCoSn.