K Uchima

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.

De Haas–van Alphen Effect and Fermi Surface Properties in Nearly Ferromagnet SrCo2P2

We grew high-quality single crystals of SrCo2P2 with the ThCr2Si2-type tetragonal structure, and clarified the Fermi surface properties by carrying out de Haas–van Alphen (dHvA) experiments and energy band calculations. SrCo2P2 is known to be a nearly ferromagnetic compound, where the magnetic susceptibility follows the Curie–Weiss law above 200 K, with the effective magnetic moment μeff = 1.72 μB/Co, but becomes almost constant below about 100 K. The electronic specific heat coefficient γ is thus relatively large, being γ = 40 mJ/(K2·mol). Detected dHvA branches possess the corresponding cyclotron effective masses \(m_{\text{c}}^{*}\), ranging from 0.87 to 7.2 m0 (m0: rest mass of an electron). The angular dependences of the dHvA frequencies are well explained by the results of full-potential linearized augmented plane wave (FLAPW) energy band calculations, revealing a multiply-connected band 25th hole Fermi surface and a compensated nearly cylindrical band 26th electron Fermi surface. It is thus conclud...

Magnetic and transport properties of EuNi(Si1-xGex)3 compounds

The magnetization M, electrical resistivity ρ, thermopower S and specific heat C of EuNi(Si1-xGex)3 compounds have been measured at temperatures from 2 to 300 K. For the compounds of EuNi(Si1-xGex)3, we obtained an effective magnetic moment of μeff ~ 7.7 μB, which is close to the divalent Eu value of μeff =7.94 μB. All compounds of EuNi(Si1-xGex)3 order antiferromagnetically. The Neel temperature TN decreases monotonously with increasing the Ge concentration x from TN=49 K for EuNiSi3 to TN=14 K for EuNiGe3. In the low temperature region below TN, anomalies corresponding to an additional magnetic phase transition into ferromagnetic state for compounds with x 0.3 were observed. The Curie temperature TC rapidly decreases with increasing x and vanishes at x ≈ 0.3. It is found that the magnetic phase transition temperatures of TN and TC in EuNi(Si1-xGex)3 are strongly connected with the change of volume induced by the atomic substitution of Si by Ge.

Effect of pressure on thermopower and resistivity of EuCo2P2

The measurements of electrical resistivity ρ and thermopower S of the single-crystalline EuCo2P2 have been performed at temperatures from 2 K to 300 K under hydrostatic pressures up to 3 GPa. The temperature dependence of ρ and S show drastic changes at the critical pressure Pc, indicating a large modification of electronic structure around the Fermi level due to a pressure-induced structural and magnetic phase transition. The magnetic phase transition temperature increases linearly with increasing pressure, and shows a sudden increase at the critical pressure Pc, which correspond to the change of magnetic state from the localized Eu(4f) sub-lattice magnetism into the itinerant Co(3d) sub-lattice magnetism.

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...

Transport properties of Heusler Compound Mn3Si under high pressure

The electrical resistivity ρ and the thermopower S of the single crystalline sample of the Heusler compound Mn3Si have been measured in the temperature range between 2 and 300 K under high pressures up to 2.2 GPa. The temperature variations of ρ and S indicate the characteristic features at the Neel temperature TN. An additional anomaly in S(T), related to the 3Q satellites in SDW, is observed at P > 1 GPa, and it disappears at P > 1 GPa. The Neel temperature, obtained from ρ(T) and S(T) curves, increases with increasing pressure. The pressure dependences of the residual resistivity and the thermopower at T = 2 K show the discontinuous changes at P ≈ 1 GPa, indicating a pressure induced phase transition.

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...

Effect of pressure on transport properties of CeIrIn5

Electrical resistivity ρ and thermoelectric power S of a heavy-fermion superconductor CeIrIn5 have been measured at temperatures from 2.0 K to 300 K under hydrostatic pressures up to 2.2 GPa. The thermoelectric power S exhibits a large positive value up to 90 μV/K, which is characteristic of heavy-fermion systems. S also shows a sharp maximum in its temperature dependence and its maximum temperature Ts, max increases with pressure, while its maximum value is constant independent of pressure. These experimental results strongly indicate that the Kondo temperature of CeIrIn5 increases by applying the pressure.

Electrical resistivity and thermopower of ErCo3 under hydrostatic pressure

The magnetic state of the Co 3d-electron subsystem of RCo3 compounds (R=rare-earth elements) with the rhombohedral PuNi3-type structure strongly depends on external parameters. In order to clarify the effect of pressure on the magnetic state of the itinerant Co 3d-electrons, we have measured the electrical resistivity and thermopower at temperatures from 2 K to 300 K under hydrostatic pressures up to 2 GPa. Both, ? and S show anomalies at critical temperature of metamagnetic transition Tm. With increasing pressure Tm, determined from the temperature-dependent resistivity and thermopower, decreases and apparently vanishes at P ? 0.7 GPa. The electrical resistivity and thermopower at low temperatures show abrupt changes at P ? 0.7 GPa, indicating a pressure-induced phase transition.

Magnetic characteristics of RPd2Si2 (R = Rare earth)

Magnetic characteristics of HoPd2Si2 and ErPd2Si2 with the ThCr2Si2-type crystal structure have been studied from measurements of magnetic susceptibility χ(T) and magnetization M(B) on the single crystals. The compound HoPd2Si2 orders antiferromagnetically below TN = 5.1 K with the magnetic easy direction of [110]. The [110] magnetization process is a three-step metamagnetic one; metamagnetic transitions at B1 = 0.9 T, B2 = 1.5 T and B3 = 3.4 T. The [100] process is a two-step one. Between [110] and [100], strong magnetic anisotropy is seen for high magnetic fields. ErPd2Si2 shows an antiferromagnetic behavior with the magnetic easy direction of [001] below TN=3.9 K. The χ(T) shows four anomalies, which may come from magnetic transitions, at Tmax = 5.1 K, TN = T1 = 3.9 K, T2 = 3.1 K and T3 = 2.2 K. The [001] magnetization process at 2 K is a three-step metamagnetic process; metamagnetic transitions at B1 = 0.5 T, B2 = 1.0 T and B3 = 1.3 T. In the basal plane magnetization processes, a very small metamagnetic transition is at 1.9 T. Crystalline electric field (CEF) analysis was performed, and anisotropic magnetic behaviors are explained. The magnetic data of the RPd2Si2 compound series are summarized.Magnetic characteristics of HoPd2Si2 and ErPd2Si2 with the ThCr2Si2-type crystal structure have been studied from measurements of magnetic susceptibility χ(T) and magnetization M(B) on the single crystals. The compound HoPd2Si2 orders antiferromagnetically below TN = 5.1 K with the magnetic easy direction of [110]. The [110] magnetization process is a three-step metamagnetic one; metamagnetic transitions at B1 = 0.9 T, B2 = 1.5 T and B3 = 3.4 T. The [100] process is a two-step one. Between [110] and [100], strong magnetic anisotropy is seen for high magnetic fields. ErPd2Si2 shows an antiferromagnetic behavior with the magnetic easy direction of [001] below TN=3.9 K. The χ(T) shows four anomalies, which may come from magnetic transitions, at Tmax = 5.1 K, TN = T1 = 3.9 K, T2 = 3.1 K and T3 = 2.2 K. The [001] magnetization process at 2 K is a three-step metamagnetic process; metamagnetic transitions at B1 = 0.5 T, B2 = 1.0 T and B3 = 1.3 T. In the basal plane magnetization processes, a very small metamagne...