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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.

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 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 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

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.

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.