Papers by Author: Kouichi Takase

Abstract: We investigated magnetic properties of spinel oxides Zn (Cr_1-xFe_x)₂O₄ to study element substitution effects on magnetism of spin-frustrated spinel oxides ZnCr₂O₄ and ZnFe₂O₄. The present study revealed that an antiferromagnetic order of ZnCr₂O₄ (T_N ~ 13 K) is suppressed by Fe doping. Zn (Cr_1-xFe_x)₂O₄ with x = 0.1, 0.5, and 0.7 exhibits spin-glass-like behavior below ~ 10 K. On the other hand, Zn (Cr_1-xFe_x)₂O₄ with x = 0.3 exhibits the absence of magnetic order or spin-glass-like behavior down to low temperature (3 K), which is probably due to the competition of geometrical and bond frustrations.

Abstract: We synthesized polycrystals of cobaltite spinels LiCo₂O₄, ZnCo₂O₄, and their mixed crystal (Li_1-xZn_x)Co₂O₄, and performed the magnetic susceptibility and specific heat measurements. LiCo₂O₄ with the Weiss temperature Θ_W ~ -114 K exhibits an antiferromagnetic transition at T_N ~ 30 K. On the other hand, ZnCo₂O₄ with the Weiss temperature Θ_W ~ -90 K exhibits the absence of magnetic phase transition down to low temperature (2 K), indicating the presence of strong frustration. Taking into account the absence of magnetic phase transition in the orbital-degenerate ZnCo₂O₄, it is suggested that the antiferromagnetic transition in the charge-orbital-degenerate LiCo₂O₄ is driven by the charge degree of freedom. Furthermore, the magnetic properties of (Li_1-xZn_x)Co₂O₄ suggest that the antiferromagnetic transition in LiCo₂O₄ is sensitively suppressed by diluting the charge degeneracy.

Abstract: We investigate electric and magnetic properties of quasi-one-dimensional transition-metal carbides Sc₃TC₄ (T = Co, Ru, and Os), and their mixed crystals Sc₃(Co_1-xRu_x)C₄ and Sc₃(Ru_1-xOs_x)C₄. Sc₃CoC₄ exhibits successive phase transitions of charge-density-wave transition at T_CDW ~ 140 K, Peierls-like structural transition at T_s ~ 70 K, and superconducting transition at T_c ~ 5 K. Sc₃RuC₄ and Sc₃OsC₄ exhibit a phase transition at T^* ~ 220 K and 250 K, respectively, which should occur in the low-dimensional electronic structure. For Sc₃CoC₄, it is revealed by the investigation of the electric and magnetic properties of Sc₃(Co_1-xRu_x)C₄ that the phase transitions at T_CDW, T_s, and T_c exhibit different robustness against Ru doping. For Sc₃RuC₄ and Sc₃OsC₄, it is revealed by the investigation of the electric and magnetic properties of Sc₃(Ru_1-xOs_x)C₄ that an identical kind of phase transition occurs at T^*. Additionally, the present study reveals that the phase transition at T^* in Sc₃RuC₄ and Sc₃OsC₄ is inherently different from the phase transitions at T_CDW, T_s, and T_c in Sc₃CoC₄.

Abstract: We have investigated the precise crystal structure and their temperature dependences of oxyarsenides (LaO)TAs; T = Mn, Fe, Co, as the Fe based superconductor’s parent material family using high energy synchrotron radiation x-ray powder diffraction. Lattice constants a and c decrease with decreasing temperature. Focusing the ratios of the changes normalized by room temperature lattice constants, we have found anisotropic shrinks for the superconductor’s parent material of (LaO)FeAs and the ferromagnetic metal (LaO)CoAs. The shrinkage of the lattice constant c along the stacking direction will be discussed by the temperature dependence of the divided three components of the LaO layer, TAs layer, and the interlayer distance.

Abstract: We have successfully prepared the new family of fluoride-arsenide Sr1-xNdxFeAsF (0 ≤ x ≤ 0.5), and investigated the electrical and magnetic properties. Sr1-xNdxFeAsF with x = 0.4 and 0.5 show superconductivity, and their superconducting transition temperatures are 32 and 49 K, respectively. The superconducting volume fractions of Sr0.6Nd0.4FeAsF and Sr0.5Nd0.5FeAsF are ~11 and ~ 17 %, respectively. On the other hand, the sample of Sr1-xNdxFeAsF (0 ≤ x ≤ 0.35) is not superconducting but shows a metallic conductivity. All compounds show a weak ferromagnetism with the Curie temperature of above room temperature. The origin of the ferromagnetism may be due to the tiny Fe impurity.

Abstract: TlFe2Se2-δ was synthesized by a self-flux method with excess amount of selenium. Elemental analysis using energy-dispersive X-ray spectroscopy estimates the deficiency δ at about 0.5. The magnetic susceptibility reveals an antiferromagnetic transition at TN ≈ 475 K. The resistivity exhibits a semiconducting behavior with an activation energy, EA = 0.04 eV. There is a clear anomaly in the resistivity at TN, indicating a strong interplay between magnetism and electric conduction.