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We succeeded in growing high-quality single crystals of pyrite-type cubic compounds CoSe2 and CoS2 using a transport agent of CoBr2 and measured the electrical resistivity, specific heat, magnetic susceptibility, magnetization, and the de Haas–van Alphen (dHvA) effect. We confirmed that CoSe2 is an exchange-enhanced paramagnet revealing a broad maximum at around 50 K in the temperature dependence of the magnetic susceptibility. The electronic specific heat coefficient is moderately large, γ = 18 mJ/(K2mol). On the other hand, CoS2 is a ferromagnet with a Curie temperature T C = 122 K and an ordered moment μ s = 0.93 μB/Co. The γ value of 21 mJ/(K2mol) in CoS2 is slightly larger than that of CoSe2. A large ordered moment, together with a large γ value, is a characteristic feature in CoS2 because CoS2 is a half-metallic spin state in the ferromagnetic state. Correspondingly, we detected a main dHvA branch with a large cyclotron effective mass of 13m 0 in the dHvA experiments. The detected dHvA branches in CoS2 and CoSe2 are discussed on the basis of the results of energy band calculations, revealing a broken four-fold-symmetry in the angular dependence of the dHvA frequency.

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.

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.

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.

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.

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.

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.

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.

Electrical resistivity ? and thermopower S of Y1?xRxCo2 (R=Gd, Tb, Dy, Ho and Er) Laves phase alloy systems were measured at temperatures from 1.5 K to 300 K in magnetic fields up to 15 T and under hydrostatic pressure up to 2 GPa. We show that there is a universal linear relation between the pressure and magnetic field derivatives of the resistivity, d?/dP and d?/dB, with gradient, determined by pressure derivative of the critical metamagnetic field of the cobalt 3d electron system. A similar scaling behavior was found for the thermopower dependencies on pressure and alloy composition.

We have developed a set-up with modified "seesaw heating method" for the thermopower measurement under pressures P up to 3 GPa at the temperature range between 2 K and 300 K. By using this set-up, the thermopower and electrical resistivity of the single crystalline YbMn2Ge2 under high pressure were measured with enough accuracy. S(T) curve shows the characteristic feature at the magnetic transition in all pressure range, while no evidence of the magnetic phase transition is observed in ρ(T) at P > 1.25 GPa. The measurement results indicate that the simultaneous measurement of the thermopower and electrical resistivity is a useful tool to study the pressure-induced phase transitions.

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.

The electrical resistivity ? of the Laves phase Y1-xRxCo2 compound system has been measured in magnetic fields up to 10 T and under pressures up to 8 GPa at temperatures from 1.5 to 300 K. The anomalous behavior of residual resistivity has been observed in a region x < xa, where x a is a critical concentration between inhomogeneously and homogeneously ordered phases, and which has a maximum at xc where Tc ? 0 with a mean field acting on Co sub-lattice is equal to the itinerant Co metamagnetic critical field Bc. In xc < x < xa, the magneto-resistivity and pressure resistivity are anomalously large with positive sign. However, in the paramagnetic region for x < xc, they are anomalously large but with negative sign. The anomalous behavior is attributed to the s-d scattering of conduction electrons due to statistically disordered Co magnetization. Those phenomena can be explained by a new scattering model of [Two magnetization Nordheim model for randomly distributed Co sites] introduced by us.

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.