Kiichi Okuda

Single High-Tc Phase Region of the Bi-Pb-Sr-Ca-Cu-O System

The phase diagram in the superconducting Bi1.68PbvSrxCavCuzOw(v=0.28, 0.32) system was investigated by means of X-ray diffraction and electrical resistivity for samples calcined in air. The results revealed that the phase was classified into four regions, a single 110 K-phase region, two types of two-phase regions (110 K phase+Ca2CuO3 and 110 K phase+80 K phase) and a three-phase region (110 K phase+Ca2CuO3+80 K phase). The single 110 K phase with a Tc of about 106–108 K exists in a fairly wide composition range, especially in the Bi1.68Pb0.32Sr1.75CayCuzOw system, 1.75y1.85 and 2.65z2.85. This means that the single 110 K phase can be easily obtained, even by the usual calcination in air in the Bi-Pb-Sr-Ca-Cu-O system.

g-Shift in Low Dimensional Antiferromagnets at Low Temperatures

Anomalous g -shifts in low dimensional antiferromagnets, copper benzoate Cu(C 6 H 5 COO) 2 3H 2 O and copper tetra-amine sulfate Cu(NH 3 ) 4 SO 4 H 2 O (CTS) were observed mainly below 1 K. These compounds show positive g -shifts as temperature decreases. The results in copper benzoate are qualitatively explained by the theory recently developed by Nagata who introduced the one-dimensional short range order effect between inequivalent spins. The shifts in CTS are discussed from the two-dimensional point of view.

Electron Spin Resonance and Nuclear Anisotropy in Dilute Alloys

Electron spin resonance (ESR) of manganese in copper and silver metals was investigated with a special interest to an anomalous line shift observed in the so called antiferromagnetic region. The shift was found to be nearly inversely proportional to temperature. This fact gives rise to a possibility that the effective anisotropy may come from the nuclear spin polarization as was first observed in antiferromagnetic KMnF 3 . To check this possibility, and ENDOR study of the Ag-Mn dilute alloys was done and the effect of the nuclear anisotropy was verified by the result that the ESR signal shows a slight shift when the nuclear spin system is under saturation with the frequency between 240 and 340 MHz. The hyperfine constant | A | was determined as 40×10 -4 cm -1 which agrees with the data given by the γ-ray polarization experiment below 1°K. It is also found that the shift is much enhanced under the presence of the third element, such as Fe, Co, Ni, and Pd atoms.

Electron Spin Resonance in the Itinerant-Electron Helical Magnet MnSi

Electron spin resonance in MnSi single crystals was observed both above and below the transition temperature T N . The experimental results below T N were explained by applying the helical spin resonance theory developed by Copper and Elliott and the effective exchange fields { J ( q 0 )- J (0)} S / g µ B and { J (0)- J (2 q 0 )} S / g µ B were obtained as 3.0 kOe and 1.0 kOe, respectively, where q 0 is the screw vector. A dynamical anisotropy field should be taken into account to explain the experimental results. The paramagnetic resonance intensity above T N decreases strongly as temperature increases.

Magnetic Properties of Layered Compound MnPS3

Magnetic properties of a layered compound MnPS 3 were investigated by means of magnetic susceptibility, high field magnetization and electron spin resonance. Temperature and angular dependence of EPR line width are characteristic of 2D-antiferromagnet. The main exchange interaction in the a b -plane is estimated to be J / k =- 9.5±0.2 K. High field magnetization process and AFMR measurements at low temperatures below T N =78 K showed a spin-flop transition at H c =38.0 kOe. They are satisfactorily explained by assuming the uniaxial anisotropy energy K =1.52±0.10×10 5 erg/cm 3 perpendicular to the a b -plane.

Antiferromagnetic Resonance in Copper Benzoate below 1 K

A linear chain antiferromagnet copper benzoate Cu(C 6 H 5 COO) 2 ·3H 2 O was investigated by antiferromagnetic resonance. The Neel temperature was determined as 0.76 K, A clear coexistence of para- and antiferromagnetic resonances was found. The antiferromagnetic resonance was analyzed by introducing the isotropic exchange, symmetric and antisymmetric anisotropic exchange, dipolar and dissimilar spin Zeeman energies. A large spin contraction due to one-dimensional spin fluctuation was found.

Antiferromagnetic Resonance in TMMC below 1 K

Antiferromagnetic resonance in a linear chain antiferromagnet (CH 3 ) 4 NMnCl 3 was observed below 1 K. The experimental results support a spin structure model of the easy plane type in the c -plane. However, the spin configuration in the plane is not determined, and the observed three resonance lines are difficult to explain satisfactorily. The zero field spin wave energy was estimated to be about 0.8 cm -1 . A temperature dependent shift of the resonance point is found at low temperatures, and it was attributed to the nuclear hyperfine anisotropy shift.

Torque Study of Layered Superconducting OxideBi2Sr2CaCu2O8+δ Single Crystal

Magnetic anisotropy of layered superconducting oxide Bi 2 Sr 2 CaCu 2 O 8+δ single crystal was investigated by torque measurements at 77 K. A hysteresis loop with a sharp cusp was found for the magnetic fields below about 6 kOe close to the ab -plane. It can be satisfactorily explained by assuming that the component of diamagnetic moment perpendicular to the ab -plane is induced by the applied external field. In fact, the field and angular dependence of the cusp is ruled well by the relation H cos θ= H c P , where H c P is the critical field perpendicular to the ab -plane at which the flux-pinning decreases abruptly with increasing field.

Observation of anisotropic pinning effect in Bi2Sr2CaCu2O8+δ single crystals

Abstract The anisotropic pinning effect of magnetic flux lines in Bi 2 Sr 2 CaCu 2 O 8+° single crystals was investigated by torque measurements in a temperature range from 4.2 to 100 K. A sharp cusp in angular dependence of the torque was found near H · ab -plane between 15 and 82 K. The magnetic field dependence of the angle yielding the cusp ϑ c1 is clearly determined by the field component perpendicular to the CuO 2 planes H cP1 obeying H cos ϑ c 1 = H cP 1, revealing the intrinsic flux trapping between the CuO 2 planes and the start of the stepwise penetration at H cP1 . In addition, extra cusps were observed in the torque curves in a narrow temperature range from 21 to 40 K, suggesting a certain change in the vortex state. The relation between fields and angles to give extra cusps ϑ c2 also well fitted the equation H cos ϑ c 2 = H cP 2 . The temperature dependence of H cP1 and H cP2 was determined and the phase transition of the vortex state is discussed.

Upper Critical Field Measurements by Pulsed Magnetic Fields in High-Tc Supoerconducting Oxides

Upper critical fields of high-Tc superconducting oxides, La1.85Sr0.15CuO4-δ, Y0.4Ba0.6CuOy and YBa2Cu3O7-δ, were measured by magnetoresistance method under a pulsed magnetic field up to 60 T in a temperature region 4.2 K to Tc. The Hc2 (0) defined by the midpoint of the resistive transition are estimated to be 40, 90 and 148 tesla, for La1.85Sr0.15CuO4-δ, Y0.4Ba0.6CuOy and YBa2Cu3O7-δ, respectively.