Shuichi Kawamata

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.

Antiferromagnetic long range order of the S = 12 linear chain cuprate Ca2CuO3

Abstract Neutron scattering measurements have been performed on a single crystal of Ca 2 CuO 3 , an S = 1 2 quasi-one-dimensional (ID) antiferromagnet. A three-dimensional (3D) antiferromagnetic long range order was observed below 8 K with an antiparallel Cu spin alignment within the CuO chain and between the nearest neighbor spins on the different chains. The anomalously small ordered moment of about 0.05 μ B might originate from the large energy scale of the spin fluctuation.

Magnetic torque measurements on superconducting λ-(BETS)2GaCl4

Abstract The magnetic torque of a quasi two-dimensional superconductor λ-(BETS) 2 GaCl 4 ( T c =5.5 K) is measured at T >1.5 K and H

3p - 3d resonant photoemission study of at low temperature

We have studied the Co 3d state of in the temperature range 20 - 110 K by using resonant photoemission spectroscopy in the Co photoabsorption region. By raising the sample temperature from 20 to 110 K, the Co 3d derived emission near the top of the valence band is transferred to higher binding-energies, and the relative peak intensity of two peaks in the satellite is changed. This is interpreted by a spin-state transition of the Co ion.

Magnetic torque ofκ-(BEDT-TTF)2Cu[N(CN)2]Br single crystal

Magnetic torque of the single crystal of a layered superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br has been measured as a function of field direction with respect to the b-axis under several magnetic fields H up to 8 kOe in the temperature range from 1.5 to 20 K. Smaller value of the anisotropy parameter γ = 180 is obtained for Cu[N(CN)2]Br salt than our previous report γ = 350 for κ-(BEDT-TTF)2Cu(NCS)2. The irreversible region in H — T phase diagram for Cu[N(CN)2]Br salt with smaller γ is larger than that for Cu(NCS)2 salt. A sharp cusp due to the intrinsic pinning is found near H∥ac-plane as in the case of our previous report on other layered superconductors.

Successive Magnetic Transitions and Large Reduction of Resistivity with Ferromagnetic Ordering in UPdGe

The electrical resistivity and magnetization of UPdGe have been measured in magnetic fields up to 60 kOe. We have found for the first time that the localized U moments enter a ferromagnetic state below 33 K following the spin-periodic ordered state below 55 K. A decrease of the resistivity is observed at both magnetic phase transitions. In the spin-periodic state, the large negative magnetoresistance accompanied by metamagnetismlike transition is observed. The ferromagnetic order reduces the electrical resistivity to a greater extent than the spin-periodic order. The results indicate that the effect of the magnetic superzone, as well as the effect of the magnetic scattering, is important when the magnetic ordering changes.

ESR study on α-(BEDT-TTF)2KHg(SCN)4 single crystal

Abstract X-band ESR measurements have been carried out on a α-(BEDT-TTF) 2 KHg(SCN) 4 single crystal at temperatures between 1.5 and 30 K. The principal axis and values of g -tensor are determined. The directions of g 1 = 2.0014, g 2 = 2.0029 and g 3 = 2.0128 are near the directions of the c -, a - and b -axis, respectively. Angular dependence of the effective g -value in the g 1 g 2 -plane is much smaller than that in the g 1 g 3 - and g 2 g 3 -planes. The splitting of resonance line appears below 4.2 K. While the line width of one split line increases with decreasing temperature down to 1.5 K, that of another one is almost independent of temperature. The splitting seems to be related to the SDW state, which is suggested to exist below 8 K.

Oriented Bulk Consolidation of Bi–Sr–Ca–Cu–O by Shock-Loading Method

Shock compaction experiments were carried out for Bi2Sr2CaCu2Oy particles sorted to 46–74 µ m and packing density of 50–82% at a shock pressure of 8 GPa. Bulk density reached 95% by shock compaction independent of the porosity. The most effective orientation was achieved by shock loading for the packing density of 67% and with post annealing at 870°C. The texture of the shocked plus postannealed specimen showed crystal growth not only within the c-plane but also along the c-axis. The Meissner volume fraction of the shock compacted specimen decreased with decreasing packing density but the onset temperature did not change. The magnetic torque of shock plus annealed specimen showed the alignment of particles.

Magnetic phase transitions in UNi1−xPdxGe and UPd1−yPtyGe

Abstract Magnetizations were measured for both UNi 1− x Pd x Ge ( x = 0.0, 0.25, 0.5, 0.75) and UPd 1− y Pt y Ge( y = 0.0, 0.25, 0.5,0.75, 1.0) systems. UNi 1−x Pd x Ge shows a sinusoidal magnetic transition with T N ≃ 55 K followed by fe rromagnetic trans-formation below 40 K. Pd rich UPd 1−y Pt y Ge undergoes a ferromagnetic state with T c lower than 60 K, while the Pt rich UPd 1−y Pt y Ge compounds reveal cycloidal magnetic transition around 50 K. The mechanism by which various magnetic phases appear in UTGE (T = Ni, Pd, Pt) is considered to be due to competition of the exchange interactions in the bc -plane accompanied by a change in transition metals.