Kiyoharu Uchima

Transport properties of Cu1−xZnxIr2S4 spinel compounds

Abstract Resistivity ρ and thermopower S of spinel compounds Cu 1− x Zn x Ir 2 S 4 with x T MI which decreases with increasing x . In the low-temperature insulating phase, the resistivity of the compounds follows to the relation ρ∼ exp (T ∗ /T) 1/2 , suggesting Efros–Shklovskii variable range hopping conductivity mechanism. The inverse of the effective activation energy 1/T ∗ , as it was obtained from ρ measured under hydrostatic pressure ( P ) for x =0, decreases linearly with P supporting the Efros–Shklovskii model. Thermopower S is positive in the insulating phase. It increases with temperature having a maximum somewhat below T MI , and abruptly decreases at T MI . In the metallic phase S is positive and linear in T up to at least 900xa0K. The maximum value of S in the insulating phase is about 4 times large for x =0.1 compared to x =0, and it quickly decreases with increasing x . It is found that the transport properties of CuIr 2 S 4 are strongly dependent on the sample preparation conditions.

Thermopower of ErCo2 in magnetic fields up to 15 T

Abstract Thermopower S and resistivity ρ of cubic Laves phase compound ErCo2 were measured from 2 to 300xa0K in magnetic fields up to H=15xa0T. ErCo2 shows a first-order magnetic transition from paramagnetic to ferrimagnetic state at Curie temperature Tc≈32xa0K, where the sharp jump in S(T) curve was observed with increasing temperature. Tc increases with increasing H. The magnitude of the jump of S(T) at the first-order magnetic transition increases with increasing H in magnetic field up to 3xa0T. At higher fields, S(T) curve shows a double transition due to separation Er and Co ordering.

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 300u2009K under magnetic fields up to 15u2009T. 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.

Pressure effect on thermopower of Y1-xGdxCo2Y1-xGdxCo2 alloy system

Thermopower of Y1-xGdxCo2Y1-xGdxCo2 pseudobinary compounds has been measured at temperatures from 1.5 to 300xa0K under hydrostatic pressure up to 2xa0GPa and in magnetic field 0–15xa0T. In the inhomogeneous and paramagnetic regions of the phase diagram the main contribution to the electronic transport is related to the strong static magnetic fluctuations, which arise due to interplay of structural disorder within Gd-sublattice and Co-3d itinerant electron metamagnetism. This complex magnetic disorder brings about novel transport phenomena, such as anomalous positive magnetoresistance found in ferrimagnetic state of the alloys. The low-temperature thermopower is almost independent of alloy composition in the ferrimagnetic range of the phase diagram (x>0.3x>0.3) indicating that the alloying does not change electronic structure of the compounds in a close vicinity of Fermi energy. However, the thermopower shows substantial variation with the composition in the inhomogeneous and in the paramagnetic regions of the phase diagram reflecting evolution of the magnetic structure with the composition.

Pressure effect on thermopower of Y1-xGdxCo2 alloy system

Thermopower of Y1-xGdxCo2Y1-xGdxCo2 pseudobinary compounds has been measured at temperatures from 1.5 to 300xa0K under hydrostatic pressure up to 2xa0GPa and in magnetic field 0–15xa0T. In the inhomogeneous and paramagnetic regions of the phase diagram the main contribution to the electronic transport is related to the strong static magnetic fluctuations, which arise due to interplay of structural disorder within Gd-sublattice and Co-3d itinerant electron metamagnetism. This complex magnetic disorder brings about novel transport phenomena, such as anomalous positive magnetoresistance found in ferrimagnetic state of the alloys. The low-temperature thermopower is almost independent of alloy composition in the ferrimagnetic range of the phase diagram (x>0.3x>0.3) indicating that the alloying does not change electronic structure of the compounds in a close vicinity of Fermi energy. However, the thermopower shows substantial variation with the composition in the inhomogeneous and in the paramagnetic regions of the phase diagram reflecting evolution of the magnetic structure with the composition.

Transport properties of Y1−xRxCo2 (R=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 300u2009K under magnetic fields up to 15u2009T. 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.

Field effect on itinerant electron magnetism of Y1−xErxCo2 compounds

Abstract Thermopower S and electrical resistivity ρ of cubic Laves phase pseudo-binary compounds Y1−xErxCo2 were measured from 2 to 300 K in magnetic fields up to 15 T . S and ρ show a strong field dependence in a vicinity of the magnetic ordering temperature. The reduction of the exchange magnetic field Bexc acting on Co 3d electrons by Y substitution for Er results in a separation of magnetic transition temperatures of Er and Co subsystems in Y0.4Er0.6Co2. The collapse of the itinerant Co 3d moments of Y0.4Er0.6Co2 is induced by applying external magnetic field about 10 T .

Effects of Pressure and Magnetic Field on Transport Properties of EuCo 2 P 2

The measurements of the electrical resistivity ρ and the thermopower S of single crystal EuCo2P2 have been performed at temperatures from 1.5 to 300 K under hydrostatic pressures up to 3 GPa and in magnetic fields up to 10 T. The temperature dependences of ρ and S show drastic changes at a critical pressure Pc, which indicates a large modification of the electronic structure around the Fermi level due to the valence transition from Eu to Eu. In the low-pressure phase with magnetic Eu and nonmagnetic Co, the magnetic field dependences of ρ and S show sudden decrease at B ≈ 7 T, implying a metamagnetic transition. On the other hand, no drastic change in ρ(B) and S(B) curves is observed in the high-pressure phase with nonmagnetic Eu and magnetic Co.

Pressure effect on thermopower of alloy system

Thermopower of Y1-xGdxCo2Y1-xGdxCo2 pseudobinary compounds has been measured at temperatures from 1.5 to 300xa0K under hydrostatic pressure up to 2xa0GPa and in magnetic field 0–15xa0T. In the inhomogeneous and paramagnetic regions of the phase diagram the main contribution to the electronic transport is related to the strong static magnetic fluctuations, which arise due to interplay of structural disorder within Gd-sublattice and Co-3d itinerant electron metamagnetism. This complex magnetic disorder brings about novel transport phenomena, such as anomalous positive magnetoresistance found in ferrimagnetic state of the alloys. The low-temperature thermopower is almost independent of alloy composition in the ferrimagnetic range of the phase diagram (x>0.3x>0.3) indicating that the alloying does not change electronic structure of the compounds in a close vicinity of Fermi energy. However, the thermopower shows substantial variation with the composition in the inhomogeneous and in the paramagnetic regions of the phase diagram reflecting evolution of the magnetic structure with the composition.