Hirofumi Kawanaka

Effect of Hydrogen Absorption on Superconductivity in YBa2Cu3O6.91 and GdBa2Cu3O6.89

Measurements of lattice constants and ac susceptibility have been performed on YBa2Cu3O6.91Hx and GdBa2Cu3O6.89Hx with x<2.0. The lattice constant a increases with increasing x, whereas the parameters b and c decrease slightly with x up to x=0.8. An orthorhombic to tetragonal transition occurs at x=0.80 for both the hydrides, after which the parameter c becomes almost constant for x up to x=1.8 in spite of the increase of the parameter a with x. The superconducting transition temperature Tc remains unchanged with x in the Y hydride system, while Tc decreases slightly with x up to x=1.0 in the Gd hydride system. The diamagnetic signal is weakened by hydrogenation for both hydrides.

Anomalous magnetic, transport and thermal properties in the half-metallic magnet UNiSn

Measurements of magnetic susceptibility, electrical resistivity, thermoelectric power and thermal expansion have been performed on the half-metallic magnet UNiSn. A second-order transition from a paramagnetic semiconductor to an antiferromagnetic metal takes place at T N =47 K without any significant lattice deformation. Unlike in the usual antiferromagnets, the curve of magnetic susceptibility sharply turns up at T N . Magnetic fields up to 13.5 T have no significant influence either on T N or the semiconducting energy gap. On the contrary, hydrostatic pressure increases T N at a rate of d T N /d p =0.50 K/kbar, but suppresses the gap energy at a rate of d E g /d p =-0.6 meV/ kbar. Thermoelectric power has an extremely sharp peak at 42 K, which is 5 K lower than T N .

Electronic state of Fe in double perovskite oxide Sr2FeWO6

The magnetic properties of double perovskite oxide Sr 2 FeWO 6 have been reported. The magnetic susceptibility and Mossbauer effect of 57 Fe show that this compound is an antiferromagnet with T N =37 K. The Mossbauer parameters below ∼20 K are the center shift of +1.2 mm/s relative to metallic iron, the quadrupole splitting of 1.9 mm/s and the hyperfine field of ∼110 kOe. The quadrupole splitting has a strong temperature dependence. From these data, we conclude that Fe in Sr 2 FeWO 6 is in the Fe 2+ high-spin state, while the hyperfine field seems to be quite small. The cell volume shows a large increase compared to other Sr 2 FeTO 6 ( T= Mo, Re, etc.), which is in the Fe 3+ high-spin state. These results suggest that these compounds have a strongly coupled charge and lattice systems.

Magnetic Structure of the Half-Metallic Magnet UNiSn

Neutron powder diffraction studies have been performed to study the unusual magnetic ordering in half-metallic UNiSn with the cubic MgAgAs-type structure. The result indicates that the magnetic structure of UNiSn is a type-I antiferromagnetic structure with the ordered uranium moment of 1.55±0.10 µ B . The Neel temperature is 46±2 K, which coincides with the transition temperature from semiconductor to metal in electrical conductivity. We therefore propose that the development of the type-I antiferromagnetism UNiSn strongly suppresses antiferromagnetic spin fluctuations and leads to disappearance of the semiconducting energy gap.

Anisotropic hybridization in a heavy-fermion compound UPdIn with double magnetic transitions

Magnetic and transport properties of a heavy-fermion compound UPdIn with a ferromagnetic moment of 0.30μ B per U atom have been studied on the single crystals. A strong anisotropy is observed in the magnetization at 4.2 K, susceptibility and resistivity along the a and c -axes of the hexagonal Fe 2 P-type structure, suggesting highly anisotropic 5f-ligand hybridizations.

Metal–insulator transition in Fe-substituted SrRuO3 bad metal system

The effect of Fe substitution in the bad metal SrRuO3 perovskite was studied by resistivity, magnetic susceptibility and Mossbauer spectroscopic measurements. The conduction behavior changed from metallic for xFe≤0.1 to non-metallic for xFe>0.26. In the intermediate concentration range, 0.1<xFe<0.26, a resistivity minimum was observed as a function of temperature. The magnetic properties showed a change from ferromagnetic to paramagnetic for xFe≥0.3. There was an enhancement in quadrupole splitting deduced from 57Fe Mossbauer spectra in going from compositions xFe<0.3 to xFe≥0.3 suggestive of a change in screening of ionic charge at this composition. These observations show that the change in conduction behavior of this system may be related to the initial small mean free path of the bad metal system as well as a change in conduction electron density brought about by substitution of trivalent Fe for quadruvalent Ru.

Antiferromagnetic Order in Disorder-Induced Insulating Phase of SrRu1-xMnxO3 (0.4 ≤x ≤0.6)

We have performed powder neutron diffraction measurements on the solid solutions of SrRu 1- x Mn x O 3 , and found that the itinerant ferromagnetic order observed in pure SrRuO 3 changes into the C-type antiferromagnetic (AF) order with nearly localized d electrons in the intermediate Mn concentration range between x =0.4 and 0.6. With increasing x , the AF moment is strongly enhanced from 1.1 µ B ( x =0.4) to 2.6 µ B ( x =0.6), which is accompanied by the elongation of the tetragonal c / a ratio. These results suggest that the substitution of Mn for Ru suppresses the itinerant characteristic of the d electrons, and induces the superexchange interaction through the compression in the c plane. We have also found that the magnetic and transport properties observed in our tetragonal samples are similar to those of recently reported orthorhombic ones.

Stractural and Magnetic Phase Transitions in a New Heavy-Fermion Compound UPd2In

Structural, magnetic, transport and thermal properties of UPd 2 In with a Heusler-type structure have been investigated. The temperature variations of thermal expansion and of X-ray diffraction pattern indicate a first-order structural transition at T s ≃180 K. The significant change in both the magnetic susceptibility and thermoelectric power near T s indicates that the uranium 5 f electrons play an important role in the structural transition. This compound undergoes an antiferromagnetic ordering at 20 K. The extrapolated value of C / T , specific heat divided by temperature, is 0.20 J/K 2 mol at 0 K; thus UPd 2 In is an antiferromagnetic heavy-fermion compound

Physical and Structural Properties of Ternary Uranium Compounds in the U–Ni–Sn and U–Ni–In Systems

New ternary uranium compounds of UNi 2 Sn and UNi 2 In have been prepared and found to crystallize in the MgCu 2 Al-type structure. On these compounds and UNi 4 Sn, UNi 4 In and U 3 Ni 3 Sn 4 with cubic structures, we report the results of electrical resistivity, thermoelectric power, magnetic susceptibility and specific heat measurements. Of the five compounds, only U 3 Ni 3 Sn 4 exhibits a moderately heavy-fermion behavior at low temperatures. The other four compounds, where the distance between the U atom and the nearest-neighboring Ni atom is almost the same as in U 3 Ni 3 Sn 4 , are, however, enhanced Pauli paramagnets. No compounds show magnetic or superconducting transition down to 1.6 K.

Magnetic properties of Ho2Ru2O7 and Dy2Ru2O7 pyrochlores

Abstract Magnetic measurements were carried out on Ho2Ru2O7 and Dy2Ru2O7 pyrochlores to study the possibility of spin ice type magnetism in these systems. Curie Weiss law fits to inverse susceptibility data in the temperature range 200– 350 K gave peff=9.60, θ=−4 K for the Ho and peff=10.54, θ=−8.4 K for the Dy system. The saturation magnetization at 2 K was only half the value expected for the ground-state configurations of the rare earth ions. The effective nearest neighbor interaction was estimated to have small positive values for both the systems. This suggests that these pyrochlore systems have a magnetic behavior similar to the other spin ice systems despite the presence of the small moment bearing 4d Ru ions which themselves order into antiferromagnetic state at an elevated temperature of about 100 K .