Yoshinao Takaesu

Transport Properties of Heusler Compounds Fe3-xVxAl

The electrical resistivity ρ and thermopower S of Heusler compounds Fe 3- x V x Al have been measured at the temperature range from 1.5 to 300 K at magnetic fields up to 15 T. The measurements of ρ and S in the high temperature range up to 800 K have been performed. ρ and S show a semiconducting behavior at high temperatures, and the obtained experimental energy gap increases almost linearly with increasing V concentration x . S is positive for x <1.0 and negative for x ≥1.0 at all measured temperatures, indicating a drastic change of major carriers at x =1.0. The electron scattering due to the magnetic fluctuations of Fe clusters has a dominant effect on the behavior of ρ of the magnetic compounds at low temperatures. On the other hand, S is not sensitive to the magnetic state and the external magnetic field, which is related to the energy dependence of magnetic scattering at the Fermi level.

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.

Weak itinerant ferromagnetism in Heusler-type Fe2VAl0.95

We report measurements of the magnetic, transport, and thermal properties of the Heusler-type compound Fe2VAl0.95. We show that while stoichiometric Fe2VAl is a nonmagnetic semimetal a 5% substitution on the Al site with the 3d elements Fe and V atoms leads to a ferromagnetic ground state with a Curie temperature TC=33±3 K and a small ordered moment μs=0.12μB/Fe in Fe2VAl0.95. The reduced value of the ratio μs/μp=0.08, where μp=1.4μB/Fe is the effective Curie-Weiss moment, together with the analysis of the magnetization data M(H,T), show magnetism is of itinerant nature. The specific heat shows an unusual temperature variation at low temperatures with an enhanced Sommerfeld coefficient, γ=12 mJ K−2 mol−1. The resistivity, ρ(T), is metallic and follows a power law behavior ρ=ρ0+ATn with n≈1.5 below TC. With applying pressure, TC decreases with the rate of (1/TC)(dTC/dP)=−0.061 GPa−1. We conclude substitution on the Al site with Fe and V atoms results in itinerant ferromagnetism with a low carrier density.

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.

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.

Transport properties of Y1−xRxCo2Y1−xRxCo2 (R=ErR=Er, Ho) in magnetic field

Thermopower S and resistivity ρρ of Y1−xRxCo2Y1−xRxCo2 (R=ErR=Er, Ho) compounds have been measured in the temperature range from 1.5 to 300 K under magnetic fields up to 15 T. Strong enhancement of resistivity and fundamental changes in temperature variation of thermopower are observed at low temperatures in the compounds within the composition range where uniform Co 3d magnetization collapses. The magnetic state of Co 3d electrons has a dominant effect on the characteristic behavior of S and ρρ in these compounds.

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.