Tatsuhito Nobata

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.

Thermal conductivity and thermoelectric power of Yb–Au system

Abstract We have measured the thermal conductivity and thermoelectric power of YbAu2 and YbAu3. Both compounds are characterized as an antiferromagnetic Kondo lattice compound. We found that the temperature dependence of thermal conductivity is well explained by assuming the Wiedemann–Franz law. This fact suggests that electronic contribution is dominant and the same carriers are responsible for both thermal and electrical transport. Thermoelectric power shows complicated temperature dependence with plural peak structures and sign reversal at low temperature. This behavior is characteristic of Kondo lattice system and is explained by the competition between Kondo effect and antiferromagnetic interaction.

Electrical resistivity of YbInAu2 under hydrostatic pressure

Abstract Hydrostatic pressure dependence measurements of the electrical resistivity of YbInAu 2 and LuInAu 2 up to 21 kbar indicate the decrease of the magnetic resistivity maximum temperature T max and the increase of the coefficient c in − ln T term as the pressure increases. Opposite response of T max and c against pressure is explained by the consequence of increasing the exchange coupling between 4f and conduction electrons and decreasing n ( E F ). The pressure dependence of the residual resistivity is compared with the previous quasi-hydrostatic experiment.

Approach of the state from antiferromagnetic to heavy fermion in CeIn(Ag1− xCux)2 under pressure

Abstract The electrical resistivity has been studied in the Heusler alloy system CeIn(Cu 1− x Ag x ) 2 under pressure up to 20 kbar. Under pressure, the Neel temperature, T N , decreases for x = 0.75 and 0.85 but increases for N = 0.50, T max (resistivity maximum temperature) decreases for x = 0.50 and 0.75 while for x = 0.85 it increases above 10 kbar. These results are analyzed and discussed according to Doniachs Kondo-necklace model. The result shows that the system approaches to the pure heavy-fermion state (for x = 0.75 and 0.85) while it approaches to the manifested antiferromagnetic state (for x = 0.50) due to the pressure-induced hybridization.

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.