後藤 貴行

The positive hyperfine field at Co nucleus in a Heusler alloy Co 2TiGa is discussed to be not dominated by the orbital hyperfine field from a quite different point of view from former studies. Field-induced-moment nuclear coupling for 59Co via spin-orbit interaction on Co atom in Co2TiGa is discussed to be small from the observed high-field shift of the NMR of 59Co in the ferromagnetic state of only +0.83 % in contrast to the case of CoCl2.2H2O with +29%. © 2009 IOP Publishing Ltd.

The spin-gap system with a bond-disorder (CH(3))(2)CHNH(3)-Cu(Cl(x)Br(1-x))(3) is reported by experiments on macroscopic quantities of the uniform susceptibility and the specific heat to show a magnetic order at low temperatures (when the value of x is within the limited region between 0.44 and 0.87) and otherwise a gapped behavior. We have investigated microscopically the ground state of the sample with x=0.35 by muon spin relaxation (mu SR) to observe that the frequency spectrum of the spin fluctuation tends to concentrate around zero energy as lowering temperatures. This behavior was interpreted as the soft mode toward the possible magnetic phase transition at absolute zero.

The ground state of the solid solution of the two spin gap systems (CH3)(2)CHNH3CUCl3 and (CH3)(2)CHNH3CuBr3 has been investigated by H-1 NMR. The existence of a magnetic ordering in the sample with the Cl-content x = 0.85 was clearly demonstrated by a drastic splitting in a resonance line at low temperatures below T-N = 13.5 K. The observed NMR spectra in the ordered state was qualitatively consistent with the simple antiferromagnetic state. (c) 2007 Elsevier Ltd. All rights reserved.

High-field magnetic torque measurement was carried out in the spin gap system (CH3)2CHNH3CuCl3. It was observed that the magnitude of the magnetic torque $\tau$ is almost zero until the critical field and then increases rapidly, which indicates the existence of magnetic quantum phase transition from the spin-gap phase to the field-induced magnetic ordered phase. From the temperature dependence of $\tau$, a cusplike minimum indicative of the Bose–Einstein condensation (BEC) of magnons was observed for $H\perp C$-plane, while a kink anomaly was observed for $H\perp A$-plane at the field-induced transition temperature. It was found that this difference in behavior between $H\perp A$-plane and $H\perp C$-plane can be interpreted as the breaking of the magnon BEC picture due to a rotational symmetry breaking. An additional anomaly was also observed in the field-induced magnetic ordered phase at the intermediate angle of the applied magnetic field in the $A$-plane and discussed in terms of spin-flop transition, spin-reorientation transition, and spin–lattice correlations. In order to investigate spin–lattice correlations in (CH3)2CHNH3CuCl3, we performed magnetostriction measurement and observed that magnetostriction appears with the disappearance of the spin gap.

The vortex state in a single crystal of the layered organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2), where BEDT-TTF (or ET) is bis(ethylenedithio)tetrathiafulvalene, was studied by H-1-NMR. Under a low field region around 0.75 T, the vortex-glass-liquid transition was demonstrated by a diverging of the longitudinal nuclear spin relaxation rate and peak broadening in spectra. Under a high field region near the upper critical field H-c2(0)similar or equal to 7 T, the curvature of nuclear spin relaxation curves showed a drastic change at the temperature where the emergence of the quantum vortex slush state was reported. The mechanism in this curvature change was discussed in terms of the fluctuating field produced by fragments of vortex glass.

We fabricated small superconducting coils whose bobbins were made of a Dyneema fiber reinforced plastic (DFRP), a Dyneema and glass fiber reinforced plastic (DGFRP), and a glass fiber reinforced plastic (GFRP). Because Dyneema fibers in the FRP expand, we are able to control thermal expansion/contraction property in a circumferential direction of the DFRP and the DGFRP pipes during a cooling process from room temperature to low temperature. We fabricated two kinds of the DFRP and the DGFRP coils whose properties were expansion and contraction. The GFRP has always a characteristic of contraction during cool-down. We measured a mechanical loss that occurred in the coil during AC excitation of those coils. The mechanical losses decreased with increasing winding tension of the conductor in the coils according to the experimental data. And the mechanical loss of the coils whose bobbins had characteristic of expansion was considerably smaller than that of contraction. It was experimentally shown that the bobbins with expansion property during the cooling down were effective to decrease the mechanical loss of the AC coils. © 2007 IEEE.

We have measured the contact resistance between superconducting strands, which is a parameter of coupling losses in cable-in-conduit conductors. Varying the twist pitches of sub-cables in the multi-stranded cables, we prepared triplets made of three strands with four different intersection angles between the strands (0, 5, 7 and 13 degrees). The experimental results showed that the range of the surface resistances estimated by both the contact areas and the contact resistances depended on the intersection angles between the strands of the cable. When we estimate inter-strand coupling losses in the cable, we have to take the contact condition of the cable into consideration. The obtained results enlarge the database used to simulate an electric circuit model of multi-stranded conductors and to estimate inter-strand coupling losses in the conductors. © 2007 IEEE.

The antiferromagnetic ordering in the solid-solution of the two spin-gap systems (CH3)(2)CHNH3CuCl3 and (CH3)(2)CHNH3CuBr3 has been investigated by H-1-NMR. The sample with the Cl-content ratio x = 0.85 showed a clear splitting in spectra below T-N = 13.5 K, where the spin-lattice relaxation rate T-1(-1) showed a diverging behavior. The critical exponent of the temperature dependence of the hyperfine field is found to be 0.33.

Muon-spin-rotation and -relaxation studies have been performed on (CH3)(2)CHNH3Cu(ClxBr1-x)(3) with x=0.85 and 0.95, which are solid solutions of the two isomorphic spin-gap systems (CH3)(2)CHNH3CuCl3 and (CH3)(2)CHNH3CuBr3 with different spin gaps. The sample with x=0.85 showed a clear muon spin rotation under zero-field below T-N=11.65 K, indicating the existence of a long-range antiferromagnetic order. A critical exponent of the hyperfine field was obtained to be beta=0.33, which agrees with 3D-Ising model. In the other sample with x=0.95, an anomalous enhancement of the muon spin relaxation was observed at very low temperatures indicating a critical slowing down due to a magnetic instability of the ground state.

We have experimentally studied relation between mechanical losses and thermal expansion properties in superconducting coils. For the experiments, three kinds of structural materials (GFRP, DGFRP, and ZFRP) were used. A glass fiber reinforced plastic (GFRP) contracts with decreasing temperature; on the contrary, a Dyneema and grass fiber reinforced plastic (DGFRP) and a Zylon fiber reinforced plastic (ZFRP) expand with the decrease of temperature. We used sample holders made of those plastics and measured AC current losses of a Bi-2223 tape in liquid nitrogen. Even if the magnetic field applied to the tape and an AC transport current of the tape were same, the loss depended on the sample holder materials. When the sample holder is the GFRP, the loss was largest in the three kinds of plastics, and in case of the ZFRP, the loss was smallest. The ZFRP has the largest expansion property of the three materials, that is, the loss decreased with expansion of the sample holder's materials. © 2006 IEEE.

A quantum spin magnet NH4CuCl3 is a three dimensional dimer system, which shows a two-stepped magnetization plateaus at M=1/4 and 3/4. In order to investigate the spatial configuration of field-induced triplons under high-field, Cu-NMR study has been performed in the field region between the first and the second plateau. It has been found that the same numbers of the differently-oriented two dimers in a unit cell remain unoccupied by triplons. This gives a strong restriction to the model to describe the plateaus in this system.

The complex NMR properties of CoVSb are reported. On the basis of band structure calculations the compound is predicted to be a half metallic ferromagnet but magnetisation measurements suggest that the compound is a weak ferromagnet. In zero applied field at 4.2 K, three spin echo signals have been observed with peak frequencies of 30, 63 and 82 MHz, which are tentatively assigned as signals associated with antimony, vanadium and cobalt atoms. The cobalt atoms are suggested to have moments of about 0-8 mu(B). In addition three spin echo signals have been observed in swept applied fields at 4.2 K which include signals from non magnetic cobalt and vanadium sites. Thus weak ferromagnetism in CoVSb is concluded to be complex on a microscopic scale with the moment of 0.18 mu(B) per formula unit, obtained from static magnetic measurements, being the average moment of magnetic and nonmagnetic sites of cobalt and vanadium atoms. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The buckling of CuO2 plane in single crystals of La-based high-T-C cuprates LSCO (x=0.15) and LBCO (x=0.08) was directly observed by Cu-NMR. In both the cases, buckling patterns obtained by NMR disagree with those expected in the averaged structure at the vicinity of the structural phase transitions.

The Hall coefficient (RH) has been measured in 0.5% Zn-doped La2-xSrxCu0.995Zn0.005O4 under high magnetic fields up to 12 T. With decreasing temperature, R-H increases and begins to decrease below a temperature T-RH. This characteristic temperature TRH has the local maximum around x = 0.195, and this Sr-concentration coincides with that the superconducting transition temperature is slightly suppressed. This behavior is quite similar to the phenomena observed in the stripe phase in x similar to 0.12. These results suggest that the anomalous decrease of RH around x = 0.195 observed in this study is responsible for the "1/4"-anomaly [as reported by Kakinuma et al., Phys. Rev. B 59, 1491 (1999).].

The sound velocity Vs of La2-xSrxCuO4 around x = 0.220 has been measured in magnetic fields up to 12 T. The increase of Vs by applying magnetic fields was observed in case of x = 0.220, and resembles the enhancement of the increase of V, observed in the concentration of x around 0.115. The magnetic field dependence of Vs in x = 0.220 scales with that observed in x = 0.120. This result strongly suggests that the observed magnetic field effect on the sound velocity has the same origin with that in x similar to 0.115.

The quantum spin magnet NH4CuCl3 is an antiferromagnetic dimer system, which has a magnetic ground state, and shows two-stepped magnetization plateaus. The origin of the plateaus and the detailed spin structure is not well understood. We have investigated the NMR spectra of deuterated ND4CuCl3 in the field region of the first plateau. Resonance peaks of the Cl-NMR spectra are classified into two groups with different temperature dependences. This indicates the existence of magnetically inequivalent dimers which are localized in space. Rotation profiles of the 2D-NMR spectra show many small peaks, indicating that the NH4 molecules are oriented in a finite number of directions. © 2006 American Institute of Physics.

Cu-NMR study under high-field up to 17T has been performed on the three-dimensional spin dimer system NH_4CuCl_3, which shows the two-stepped magnetization plateaus. The local magnetization of Cu-site measured by NMR showed a significant deviation from the macroscopic magnetization at low temperatures, suggesting the localization of the field-induced triplons. In the high field region around 16 T, two inequivalent Cu sites are observed. Only one of the two sites shows a split in the resonance line below T_N≃3.2K, indicating the existence of the field-induced magnetic order, to which a part of spins in the system participate.

Tl-based high-T-C cuprate TlBa2Ca1-xYxCu2O7 (Tl1212) shows an anomalous electronic phase diagram, in which the non-superconducting phase persists on hole doping until x = 0.6 where T-C abruptly jumps to 96 K. Around the critical hole concentration, Tl nuclear relaxation curves follow a so-called stretched-exponential function, suggesting that the doped holes are localized in small islands. This microscopic inhomogeneity in hole distribution is considered to be the origin of anomalous suppression of the superconductivity in the lightly doped region. (C) 2003 Published by Elsevier B. V.

We have performed H-1 and Se-77 NMR measurements on a pi-d organic metal k-(BEDT-TSF)(2)FeCl4 containing paramagnetic ions Fe3+. It is found from 1 H NMR measurements that the resonance line starts to split around T-FM = 70 K, just a temperature where a ferroelectric-like transition occurs, and with further decreasing temperature, the spectrum becomes significantly broad near the metal-insulator transition T-MI = 8.3 K. From Se-77 NMR measurements, near the phase boundary between the forced-ferromagnetic metallic state and the canted antiferromagnetic insulating state at 1.6 K and 11.67 T, a single peak becomes split with two new peaks emerging at the both sides of the zero-shift. (C) 2002 Elsevier Science B.V. All rights reserved.

We have carried out Ru-99/101 and Cu-63/65 nuclear magnetic resonance experiments in order to investigate magnetic and electronic properties of the magnetic superconductor RuSr2RECu2O8 (RE = Gd, Eu and Y). The two kinds of Ru-99/101 signals ere observed in the magnetically ordered state for each system, suggesting a charge segregation of Ru5+ (S = 3/2) and Ru4+ (S = 1) ions in the RuO2 layers. The internal field at the Cu sites is revealed to be of the order of kilo Oe, indicating weak magnetic interactions between the CuO2 and RuO2 planes. The temperature dependence of nuclear spin-lattice relaxation time T-1 of Cu-63 in E = Y shows a 'spin gap' like behavior, suggesting the system is under-doped. (C) 2002 Published by Elsevier Science Ltd.

The magnetization in a lightly doped La2-xSrxCuO4 (x = 0.018) single crystal was measured. Spin-flop transition was clearly observed in the hole-doped antiferromagnetically ordered state under increasing magnetic fields perpendicular to the CuO2 plane. In the spin-glass phase below 25 K, the spin-flop transition becomes broad but the step in the magnetization curve associated with the transition remains finite at the lowest temperature. We show in this report that, at low temperature, the homogeneous antiferromagnetic order is disturbed by the redistribution of holes, and that the spatial variance of the local hole concentration around x = 0.018 increases.

The magnetization in a lightly doped La2-xSrxCuO4 (x=0.018) single crystal was measured. Spin-flop transition was clearly observed in the hole-doped antiferromagnetically ordered state under increasing magnetic fields perpendicular to the CuO2 plane. In the spin-glass phase below 25 K, the spin-flop transition becomes broad but the step in the magnetization curve associated with the transition remains finite at the lowest temperature. We show in this report that, at low temperature, the homogeneous antiferromagnetic order is disturbed by the redistribution of holes, and that the spatial variance of the local hole concentration around x=0.018 increases.

The Hall coefficient in the low-temperature tetragonal phase and the midtemperature orthorhombic phase of La1.88-yYySr0.12CuO4 (y=0,0.04) single crystals is measured under high magnetic fields up to 9 T in order to investigate the detailed behavior of the transport properties at low temperatures in the stripe phase. When the superconductivity is suppressed by high magnetic fields, the Hall coefficient has negative values in low temperatures, and the temperature region of the negative values spreads as increasing magnetic fields. This result indicates that the Hall coefficient in the stripe phase around x = 0.12 is a finite negative value, not zero.

Tl-NMR spectra and the spin-lattice relaxation rate have been measured for Tl-based high-T-C cuprate TlBa2(Y1-xCax)Cu2O7 with a wide region of the hole concentration from antiferromagnetic (x = 0) to slightly overdoped (x = 1). On doping, the superconducting transition temperature T-C is raised abruptly from 0 to 100 K at x = 0.6. In the entire superconducting region of x = 0.6-1.0, the Knight shift is temperature independent of the normal state, showing no traces of the spin-gap. The spin-lattice relaxation rate shows a spin-gap around 150 K, which is much higher than T-C. These two facts in striking contrast indicate that the lightly doped region does not exist or is confined within an extremely narrow region of Ca-concentration. (C) 2002 Elsevier Science Ltd. All rights reserved.

The cyclic properties of protium storage in Ti-Cr-V alloys with a BCC structure were investigated in order to develop the alloys with good cyclic properties. It was found that the hysteresis factor of pure V was almost unchanged but the hysteresis factor of Ti-Cr-V heat-treated alloys decreased with increasing cycle number. The plateau pressure of Ti-Cr-V heat-treated alloy in absorbing process dropped significantly with increasing cycle number up to the 20th cycle. In contrast to this, there was not any change in PCT curves of pure V metal. This could be originated from the difference in mechanical properties between V metal and Ti-Cr-V alloys.

The Hall coefficient in the low-temperature tetragonal phase and the midtemperature orthorhombic phase of (formula presented) single crystals is measured under high magnetic fields up to 9 T in order to investigate the detailed behavior of the transport properties at low temperatures in the stripe phase. When the superconductivity is suppressed by high magnetic fields, the Hall coefficient has negative values in low temperatures, and the temperature region of the negative values spreads as increasing magnetic fields. This result indicates that the Hall coefficient in the stripe phase around (formula presented) is a finite negative value, not zero. © 2002 The American Physical Society.