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We have confirmed that the tetragonal ternary compound DyIr2Si2 shows polymorphism; the ThCr2Si2-type structure as a low temperature phase (I-phase) and the CaBe2Ge2-type one as a high temperature phase (P-phase) exist. A comparative study on magnetic characteristics of the morphs was performed on the I- and P-phase single crystals in order to elucidate how magnetic properties are influenced by crystallographic symmetry. The magnetic behavior changes drastically depending on the structure. The DyIr2Si2(I) shows an antiferromagnetic ordering below TN = 30 K, additional magnetic transitions of T1 = 17 K and T2 = 10 K, and a strong uniaxial magnetic anisotropy with the easy [001] direction. The [001] magnetization shows four metamagnetic transitions at low temperatures. On the other hand, the DyIr2Si2(P) has comparatively low ordering temperature of TN1 = 9.4 K and an additional transition temperature of TN2 = 3.0 K, and exhibits an easy-plane magnetic anisotropy with the easy [110] direction. Two metamagnetic transitions appear in the basal plane magnetization processes. In both the morphs, the χ-T behavior suggests the existence of component-separated magnetic transitions. The ab-component of magnetic moments orders at the higher transition temperature TN1 for the P-phase compound, which is contrast to the I-phase behavior; the c-component orders firstly at TN. The crystalline electric field (CEF) analysis was made, and the difference in magnetic behaviors between both the morphs is explained by the CEF effects.

Magnetic measurements on the pseudo-ternary compounds Ho 1- x R x Rh 2 Si 2 (R = Y or La = Y or La) were performed in order to get information on the origin of “the successive component-separated magnetic transitions” which appear in HoRh 2 Si 2 . The lattice parameters a and c remain almost constant during changes to Y composition x , while they increase with increasing La composition x . The c / a ratios are also constant in the Y-system, and they increase with increasing x in the La-system especially for above around x = 0.4. The transition temperatures, T N1 = 29.1 K, T t = 27.3 K and T N2 =12.1 K at x = 0, decrease with increasing x . The rates of decrease for T N1 and T N2 in the Y-system accord with those in the La system below x = 0.4. The critical compositions for T N1 and T N2 are determined to be x N1 = 0.88 and x N2 = 0.59 in the La-system, respectively, and x N1 = 0.98 and x N2 = 0.75 in the Y-system. In both the systems, “the successive component-separated magnetic transitions” appear for the wide x regions. The magnetic ordered state persists in very dilute Ho-compounds in spite of much weak magnetic interactions. The effective magnetic moments are almost constant for the ordered compounds; μ eff = 10.6 ± 0.30 μ B /Ho. These behaviors are strange, indicating a strong correlation exists in these systems.

The compounds of EuNi(Si1−xGex)3 order antiferromagnetically. At the temperature TC below the Ńeel temperature TN, EuNiSi3 (x = 0) shows an additional magnetic transition into ferro-magnetic state. TN decreases monotonously with increasing the Ge composition x. The Curie temperature TC decreases rapidly with increasing x and vanishes at the critical composition x ≈ 0.3. We have measured the electrical resistivity and thermopower of EuNi(Si0.8Ge0.2)3, which is a compound near to the boundary between the ferromagnetic and antiferromagnetic ground states in the phase diagram for EuNi(Si1−xGex)3 system, under pressures up to 1.8 GPa at temperatures from 2 to 300 K. The anomalies in ρ(T) and S(T) curves of EuNi(Si0.8Ge0.2)3 are observed at TC = 16 K and TN = 34 K at ambient pressure. Both TC and TN increase linearly with increasing pressure. The temperature variations of ρ and S of EuNi(Si0.8Ge0.2)3 at P = 1.8 GPa are almost the same as those of EuNi(Si0.9Ge0.1)3 (x=0.1) at ambient pressure, revealing that the effect of pressure on TN and TC is the same as that of the increase of Si concentration. The pressure and atomic composition dependences of the magnetic transition temperatures TN and TC can be expressed by using the Gruneisen parameters. These results indicate that the changes of TN and TC are attributed to the change of atomic volume induced by the applying pressure or the atomic substitution.

We grew single crystals of a ferromagnet LaCo 2 P 2 and an antiferromagnet CaCo 2 P 2 , and clarified the magnetic properties by measuring the electrical resistivity, specific heat, magnetic susceptibility, and magnetization. For LaCo 2 P 2 , we also carried out the de Haas-van Alphen (dHvA) experiment. Detected dHvA branches are well explained by the results of energy band calculations using a full-potential linearized augmented plane wave method within the local spin density approximation, where the ferromagnetic state is assumed. The cyclotron effective masses are (2–3) m 0 ( m 0 : rest mass of an electron). The present relatively large cyclotron masses in LaCo 2 P 2 are due to ferromagnetic correlations of Co-3 d conduction electrons, which are compared with much larger cyclotron masses of (3–7) m 0 for a nearly ferromagnet SrCo 2 P 2 .

The electrical resistivity ρ and the thermopower S of Ho(Co1−xAlx)2 alloys (x = 0 to 0.2) were measured at temperatures from 2 K to 300 K in magnetic fields up to 10 T. While ρ(T) and S(T) of pure HoCo2 reveal abrupt changes at the magnetic ordering temperature TC, indicating a firstorder transition, the temperature variations of both properties across the ordering temperature in Ho(Co1−xAlx)2 alloys clearly show that the type of magnetic transition changes under substitution of Al for Co from first order to second order around x ≈ 0.06. ρ(T) of the Al-substituted samples (x = 0.15 and 0.2) has a very unusual variation at temperatures below TC: in contrast to the expected and usually-observed decrease of ρ(T), the resistivity of Ho(Co1−xAlx)2 alloys (x = 0.15 and 0.2) increases at temperatures below TC. Moreover, the magnetoresistance of these alloys is positive around TC; this is also in a sharp contrast to the usually observed negative magnetoresistance. We show that the anomalous resistivity and magnetoresistance in these alloys are related to the metamagnetic instability of the Co 3d subsystem and to the fluctuating local magnetic susceptibility due to atomic substitution. Due to this fluctuating local magnetic susceptibility, the uniform exchange field of 4f moments induces a non-uniform polarization of the 3d system, with static spatial fluctuations of the local magnetization. This static magnetic disorder gives an additional contribution (ρm) to the electrical resistance in the alloys at temperatures below TC. The degree of this disorder changes with decreasing temperature, resulting in specific variation of ρm with temperature.

The electrical resistivity ρ and the thermopower S of pseudo-binary Y1−x Nd x Co2 alloys have been measured at temperatures from 2 to 300 K under pressures up to 3.5 GPa. NdCo2 is a ferromagnet with a Curie temperature T C of 98 K. In the ferromagnetic phase, it undergoes a 4f spin-reorientation transition at T R = 40 K. At this transition, the easy direction of 4f magnetization is changed from [100] at temperatures above T R to [110] below T R. The system phase diagram T C(x, P) and T R(x, P) is inferred from the temperature dependencies of the electrical resistivity and the thermopower. The Curie temperature of the alloys, TC decreases with decreasing Nd concentration x and vanishes around x c = 0.3. Above x c, the Curie temperature decreases with increasing pressure. The spinreorientation temperature weakly depends on the composition and on the pressure until it merges with T C. There is a large region of the phase diagram, partly overlapping with the ferromagnetic phase, where the cobalt 3d electron system is non-uniformly magnetized due to spatial fluctuations of the 4f-3d exchange field related to a random distribution of Nd 4f magnetic moments over the R sublattice. In this region, large static 3d magnetic fluctuations govern the electronic transport.

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...

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 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.

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...

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.

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...

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

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.

The thermoelectric power S and the electrical resistivity ρ of a single-crystalline CeIrSi 3 with the non-centrosymmetric tetragonal crystal structure were measured under pressure up to 2.72 GPa in the temperature range from 1.5 to 300 K. The electronic state of CeIrSi 3 is turned from the antiferromagnetic state to superconducting state as a function of pressure. S exhibits a large positive value up to about 52 µV/K, which is characteristic of heavy-fermion systems. S shows a maximum in its temperature dependence due to the Kondo effect and its maximum temperature T S ,max gradually moves to higher temperatures from about 100 K (at ambient pressure) to about 120 K (at 2.72 GPa) by applying the pressure. We also observed the antiferromagnetic transition as the kink in the temperature dependence of S at pressure up to 2.12 GPa.

The electrical resistivity ρ and thermopower S of the cubic Laves phase compounds Ho(Co 1- x Al x ) 2 ( x =0, 0.15 and 0.20) have been measured at temperatures from 2 to 300 K in magnetic fields up to 10 T. The measurements of ρ under pressures up to 3 GPa have been performed in the temperature range between 2 K and 300 K. Both ρ( T ) and S ( T ) of the pure HoCo 2 reveal abrupt changes at the magnetic ordering temperature T C , indicating the first order transition. ρ( T ) of the Al substituted samples ( x =0.15 and 0.20) show anomalous behavior in the form of an increase below T C . The variations of ρ and S at T C clearly indicate that the type of the magnetic transition changes under substitution of Al for Co from the first order to the second order. The residual resistivity of Ho(Co 1- x Al x ) 2 shows a rapid increase with increasing x . T C increases linearly for x < 0.15, having a maximum at x ≈0.15 and decreases with further increase of x . The peculiar x dependence of T C and the anomalous behav...

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.