Masataka Takeda

Effect of pressure on thermopower of EuNi2Ge2

EuNi2Ge2 is antiferromagnetic below TN ≍ 30 K with an effective moment µeff ≍ 7.7 μB, indicating the 4f7 electron configuration (Eu2+) in the ground state. In order to investigate the electronic state of EuNi2Ge2, we have simultaneously measured thermopower S and electrical resistivity ρ at the temperature range between 2 K and 300 K and under pressures up to 3.5 GPa. In the pressure region of P 2.3 GPa, ρ increases with increasing temperature, and shows an anomaly in the form of a kink at the Neel temperature TN. S(T) also reveals a kink at TN. Both ρ(T) and S(T) indicate a small pressure dependence at the low pressure range. However, ρ(T) and S(T) curves in the low temperature region suddenly change their features at P ≍ 2.3 GPa, where the magnetic ordering disappears. ρ linearly decreases with decreasing temperature, and shows a sudden drop at the valence transition temperature Tv ≍ 30 K. S(T) reveals a drastic increase at Tv, changing its sign from negative to positive around 35 K, and takes maximum at T ≍ 7 K. The thermal hysteresis was clearly observed in both ρ(T) and S(T) curves around Tv.

Pressure and Magnetic Field Effects on Electrical Resistivity and Thermopower of Y1-xTbxCo2 System

The electrical resistivity ρ and thermopower S of the Laves phase Y 1- x Tb x Co 2 alloy system have been measured in magnetic fields up to 10 T and under pressures up to 8 GPa in the temperature range from 1.5 to 300 K. The residual resistivity of the alloys peaks at the critical composition x c ≈0.2 of the onset of 4f-moment long-range magnetic order in the Tb sublattice. In the magnetically ordered region of the phase diagram x > x c , an anomalous large positive magnetoresistivity and a pressure effect on resistivity are observed. The variation of the resistivity with alloy composition, magnetic field, and pressure are attributed to the scattering of conduction electrons due to magnetic static disorder within the partially ordered Co 3d itinerant subsystem. The evolution of the temperature dependence of thermopower with pressure are in agreement with this conduction mechanism.

Characteristic Fermi Surface Properties of V2Ga5, CoGa3, TiGa3, ZrGa3, and ZrAl3 with Different Tetragonal Structures

To clarify the Fermi surface properties based on the dimensionality of electronic states, we grew single crystals of V2Ga5, CoGa3, TiGa3, and ZrGa3 (ZrAl3) with different tetragonal structures and carried out de Haas–van Alphen experiments, together with full-potential linearized augmented plane wave band calculations. A nearly one-dimensional plate Fermi surface is obtained in the band calculations for V2Ga5 with a flat tetragonal structure. The corresponding single crystal is of needle shape along the tetragonal [001] direction. The obtained Fermi surfaces of CoGa3 are very similar to those of Ni3Ga with the AuCu3-type cubic structure because the crystal structure of CoGa3 is nearly cubic and the corresponding single crystal is of pyramidal shape with a flat (111) plane. The Fermi surfaces of TiGa3 are also very similar to those of YCu2Si2 with the ThCr2Si2-type tetragonal structure. The cylindrical Fermi surfaces with concave and convex parts are realized in ZrGa3 and ZrAl3 with the flat Brillouin zone...

Pressure and Substitution Effects on Transport and Magnetic Properties of Y1-xRxCo2 Systems with Static Magnetic Disorder

The electrical resistivity and thermopower of light- and heavy-rare-earth-based pseudo-binary Y1-xRxCo2 (R = Nd, Gd, and Tb) alloys are measured at temperatures from 2 to 300 K under pressures up to 3.5 GPa. The resistivity and thermopower of Y1-xRxCo2 show unusual large variations with atomic substitution and pressure in the range of x<xm, where an inhomogeneous magnetization of the Co 3d electron subsystem is observed. These results indicate that the low-temperature transport properties of Y1-xNdxCo2, as well as of Y1-xR\text{HCo2 (R\text{H = heavy rare earth) alloys, are related to conduction electron scattering due to the static magnetic disorder in the itinerant Co 3d electron subsystem. We found that there is a universal relationship between \(\mathrm{d}\ln T_{\text{C}}/\mathrm{d}P\) and \(x/x_{\text{m}}\) in Y1-xRxCo2 alloys, where \(x_{\text{m}}\) is the boundary composition, which separates the alloy phase diagram into regions with uniform and nonuniform magnetizations of the Co-3d electron subsy...

Pressure effect on transport properties of NdCo2

Electrical resistivity ρ and thermopower S of the Laves phase compound of NdCo2 has been investigated at temperatures from 2 K to 300 K. ρ has been measured under pressures up to 8 GPa and S has been measured under pressures up to 3 GPa. The magnetic transition temperature TC obtained by ρ measurement decreases with increasing pressure. The temperature Tmim where the thermopower S takes minimum at high temperature region increases linearly with increasing pressure. The high-temperature minimum of S is associated with a sharp peak in density of states related mainly to the Co 3d-electron density. Since the width of an itinerant electronic band depends on the extent of the corresponding overlapping of 3d orbitals, the pressure variation of Tmin can be attributed to the broadening of the peak width of 3d electron density of states.

Transport properties of Y1-xNdxCo2 compounds

Electrical resistivity ρ and thermopower S of light rare earth-based pseudo-binary Y1–xNdxCo2 alloys have been measured at temperatures from 2 K to 300 K and under pressures up to 3.5 GPa. The Curie temperature of the alloys, TC, determined from characteristic features in the temperature dependences of the transport properties, decreases with decreasing Nd concentration x and vanishes around xc = 0.3. The residual resistivity has a pronounced maximum at x = xc. The temperature coefficient of thermopower dS/dT at low temperature limit shows a complex dependence on alloy composition: it changes its sign from negative to positive at x 0.2, having a maximum at x = xc, and is nearly composition independent at x > 0.5. The pressure dependences of TC and ρ0 of Yo.6Ndo.4Co2 reveal the behavior similar to that observed in the Y1-xRxHCo2 (RH = heavy rare earth) alloy systems, which implies that the magnetic state of the Co-3d electron subsystem is responsible for the transport properties in the Y1-xNdxCo2 alloys.

Pressure effect on transport and magnetic properties of Nd1-xTbxCo2

The electrical resistivity and magnetization of Laves phase Nd1−xTbxCo2 compounds have been investigated at temperatures from 2 K to 300 K in magnetic fields up to 10 T under pressures up to 8 GPa. The magnetic transition temperature TC increases linearly with increasing Tb concentration x, which indicates that the 4f-3d exchange interaction energy increases linearly with increasing x. The spontaneous magnetization MS at small x decrease linearly, vanishes around x ≈ 0.35 and then increases linearly with increasing x. This behavior suggests that the Tb magnetic moment is antiparallel to that of Nd, while Co moment is almost independent of x.