Shuji Ebisu

A new ferromagnetic thiospinel CuCrZrS4 with re-entrant spin-glass behaviour

A new thiospinel CuCrZrS4 has been successfully synthesized by a solid-state chemical reaction. This CuCrZrS4 exhibits ferromagnetic properties below the Curie temperature at T c = 60 -2 K. The appearance of irreversible effect between field-cooled and zero-field-cooled magnetization is prominent below around 5 K in a magnetic field of less than 150 Oe. The ac susceptibility χAC shows a rapid decrease below about 10 K. This low magnetic-field behaviour indicates the existence of a re-entrant spin-glass phase below about 10 K. The dc magnetic susceptibility above 100 K shows Curie-Weiss behaviour with an effective magnetic moment of 3.61 μ B, which is a little less than the spin-only value of 3.87 μ B for the Cr3+ ion. The asymptotic Curie temperature θ P is approximately 65 K, which is a little higher than T c. The valence state is confirmed to be Cu+Cr3+Zr4+S4 2− on the basis of magnetic properties. The electrical resistivity ρ shows a semiconducting temperature dependence over the temperature range from 4.2 to 280 K with an activation energy of 6.84 210−3 eV in the higher temperature range from 50 to 283 K.

Superconductivity in the layered compound 2H-TaS2

Abstract The layered compound 2H-TaS 2 is one of the typical materials which lead to charge density waves. However, much less information is available on the superconducting properties of this compound. The Meissner effect in 2H-TaS 2 was studied systematically with special emphasis on low-d.c. applied fields. The susceptibility χ = M / H is quite weak and the value of χ is less than −8.0 × 10 −3 emu cm −3 above 0.4 K, at least 10 times smaller than the magnitude for perfect diamagnetism. A transition occurs over a rather wide temperature range. An indication of diamagnetism appears around 1.5 K which is much higher than the value of 0.8 K reported as the transition temperature in the earlier literature. The magnetization curves depart from linearity at fields less than 2.0 Oe at 0.4 K, which implies that the lower critical field H cl of 2H-TaS 2 is extremely small.

Neutron diffraction study of α-Gd2S3

Neutron diffraction measurements on a powder sample of α- 160 Gd 2 S 3 are performed. This material consists of a stacking of square lattice Gd1 and triangular Gd2 planes and exhibits a novel phase transition that was interpreted as being due to geometrical frustration. The magnetic unit cell is found to be the same as the chemical unit cell. The magnetic moments order parallel with each other along the b -axis. The magnetic structure in the a c plane is such that a Gd1 moment is ordered antiparallel to the nearest neighboring Gd1 moments and is coupled parallel to the next-nearest neighboring Gd1 moments. The Gd2 moment is coupled antiparallel to the nearest neighboring Gd1 moment. The temperature dependence of the order parameter is extracted and is found to be explained rather well by the result of an S = 7/2 molecular field approximation. The collinear structure of the magnetic moments in the triangular lattice and the absence of large magnetic fluctuations around the magnetic transition temperature s...

Phase Transition of a Frustrated Magnet α-Gd2S3

Magnetic susceptibility, magnetization and heat capacity measurements are performed on a single crystal sample of α-Gd 2 S 3 . From the heat capacity measurements, we find a sharp anomaly at ≃10 K ...

Superconductivity in the metallic layered compound NbTe2

Abstract The layered-structure compound NbTe2 is one of the typical materials which lead to charge-density waves. However, much less information is available on the superconducting properties of this compound. Electrical resistivity and diamagnetic susceptibility have been measured for high-quality single crystals of NbTe2. The superconducting transition is observed at Tc of 0.5 K. The transition to the superconducting state depends strongly on the current density. Susceptibility χ ∥ = M H with applied magnetic fields parallel to the two-dimensional layers in the crystal is weak and the value of χ∥ is two times smaller than the magnitude for perfect diamagnetism in a magnetic field of 0.50 Oe. The susceptibility changes with external field, even less than 0.50 Oe at 0.40 K, which implies that the lower critical field Hc1 of NbTe2 is extremely small ( H c1

Ferrimagnetic order in the mixed garnet (Y1-xGdx)3Fe5O12

= 0.5 Oe ). The experimental results of NbTe2 are compared with the data of the other layered compound 2H-TaS2.

Synchrotron X-ray Diffraction Studies of α-Gd2S3

Ferrimagnetic order in the mixed rare-earth iron garnet (Y1 −  x Gd x )3Fe5O12 system has been reinvestigated to cover the temperature range 5.0 to 700 K. The magnetization versus temperature exhibits a systematic variation with changing concentration x. The ferric-ion exchange coupling is strong enough to determine the Curie temperature ≈559 K for all the values x. The compensation temperature at which the magnetization crosses zero (shows the minimum) demonstrates the applicability of the three-sublattice model. The magnetic moment at 5.0 K indicates reasonable agreement with the relation n B = |21x − 5.0|μB for the (Y1 −  x Gd x )3Fe5O12 system. The compensation temperature decreases with decreasing x from 1.0 and reaches zero near x = 0.24. An enlargement of coercive force in the hysteresis loop for the low-field M-H curve is clearly seen in near the compensation temperature, indicating that a single domain is formed and the rotation of this single domain occurs without building up a multidomain structure.

Superconductivity and metal-insulator transition in Cu(Ir1−x Rh x )2S4

Non-resonant X-ray Bragg diffraction measurements on a small, single crystal of α-Gd 2 S 3 in zero and non zero applied magnetic fields are reported. From independent neutron diffraction measurements, the magnetic unit cell is found to be the same as the chemical unit cell. Therefore, the observation of magnetic X-ray diffraction by this compound is particularly challenging. We observe a change in the intensity around the Neel temperature in zero field, as well as around the phase boundaries at finite magnetic fields. The X-ray diffraction intensity is calculated based on the previously established magnetic structure. A reasonable amount of intensity from the interference between magnetic and charge scattering is predicted, and it provides a satisfactory account of our observations.

SUPERCONDUCTIVITY IN THE FILAMENTARY CONDUCTOR TASE3

A new thiospinel CuIr2S4 exhibits a metal-insulator (M-I) transition at 226 K, while CuRh2S4 shows a superconducting transition at 4.70 K. We present a systematic study of electrical and magnetic properties of Cu(Ir1−xRhx)2S4. TheM-I transition of CuIr2S4 is accompanied by a structural phase transition from tetragonal symmetry in insulating phase to cubic symmetry in high temperature metallic phase. With increasing Rh contentx, the sharpM-I transition shifts to lower temperature forx≦0.10. The samples show semiconductive behavior for 0.10≦0.30 between 4.2 and 300 K, and recover the metallic state forx≧0.50. The superconducting transition may occur for very close tox=1.00. Magnetic susceptibility shows the jump at theM-I transition temperature and the variation ofx leads to a systematic change of the magnetic susceptibilities, which is consistent with the electrical characteristic feature.

Drastic change of the electrical resistivity related to the novel magnetic phase transition in α-Sm2S3

Abstract Superconductivity in a quasi-one-dimensional conductor TaSe 3 has been systematically studied. Resistivity below T c (2.0 K) is strongly non-linear for current densities higher than 0.1 Amm −2 , reflecting severely restricted current paths in the crystals. The diamagnetic susceptibility χ does not exhibit any indication of an abrupt change at T c . Furthermore, χ shows no significant variation of diamagnetism, with magnetic fields parallel and perpendicular to the crystalline b -axis over the low magnetic field range of 7.2 mOe to 4.0 Oe. The diamagnetism is quite weak and the value of χ is much less than −3.0 × 10 −3 emu cm −3 even at 0.40 K. The superconducting thin filaments in the whisker crystal would have an effective radius much smaller than the London penetration depth λ and this penetration reduces the values of χ. These filaments are individually fairly well isolated above 0.40 K.