後藤 貴行

Single-pancake coils having different winding turns are fabricated. The bobbin material used for the coils is a Dyneema fiber-reinforced plastic (DFRP), which has a thermal expansion property during cooling. The influence of the DFRP's expansion on thermal strains of the coil during cooling is experimentally estimated. In the experiment, the thermal strains on the superconducting tape of the outermost turns in the coils are measured. The dependence of epoxy impregnation on the thermal strain of the coil is also experimentally estimated. According to the experimental data, the combination of impregnation and the bobbin's expansion effectively decreases thermal contraction of the outermost tape.

Longitudinal-field muon-spin-relaxation (LF-μSR) measurements in TlCu1-xMgxCl3 were carried out to investigate the spin dynamics in the non-magnetic impurity doped spin gap system. As reported before, in the case of x ≥ 0.006, LF-μSR time spectra are well fitted by the two components function, which means the phase separation to a spin frozen region and to a spin flucuating region. In this report, we focus on the spin fluctuating region, and discuss the gradual change in its magnetic properties with increasing the Mg-doping.

133Cs/7Li-NMR study was performed on a quasi-two-dimensional spin system Cs2Mn3LiF12, in which S 2 Mn atoms form undistorted Kagome lattice. We have observed a clear splitting in the Li-NMR resonance line for a single crystal at low temperatures below TN 3.6 K. The longitudinal nuclear spin relaxation rate T-1 1 of both Cs and Li-NMR showed a critical divergence at the same temperature. These observations are considered to be a microscopic evidence for an occurrence of long range magnetic order due to a weak inter-plane spin interactions.

We fabricated nonimpregnated HTS superconducting coils with a BSCCO tape. Bobbin materials in the coils are a Dyneema(R) fiber reinforced plastic (DFRP) and a glass fiber reinforced plastic (GFRP). We excited those coils with ac currents, and estimated mechanical losses. According to the measured data, the mechanical loss decreased with increase of winding tension of the coils, because strong winding tension fixed the coils tightly. Also, the mechanical loss occurred in the DFRP coil was smaller than that occurred in the GFRP coil. A thermal expansion coefficient of the DFRP is a negative value, that is, the DFRP expands with cooling down from room temperature to cryogenic temperature. The expansion of the DFRP bobbin made the winding of the coil fix tightly, and the mechanical loss decreased. From those experimental results, we think that the DFRP bobbin is useful for decreasing of mechanical losses of the ac coils. © 2002-2011 IEEE.

We experimentally study thermal stability of conduction cooled superconducting coils from a viewpoint of structural materials of the coils. The materials are Dyneema fiber reinforced plastics (DFRPs). The DFRP has properties of high thermal conduction and negative thermal expansion. We evaluated the stability of the conduction cooled coils having DFRP bobbins with changing the DFRP's thermal expansion. According to the experimental results, when the expansion became large, the stability also became better. It is expected that the DFRP is the effective heat sink bobbin material. © 2011 IEEE.

B-11-NMR experiments were performed in high magnetic fields applied along a-axis up to 17.5T to investigate the field-induced magnetic phase transitions and the magnetic structure in high magnetic fields microscopically for the single crystalline TbB4, Shastry-Sutherland-type frustrated antiferromagnet. It was found that the field-swept B-11-NMR spectra observed at low magnetic field changes drastically at H-C = 15.9 T, where the magnetization jump occurs. Based on a simple model of four-spin cluster and the classical dipole-dipole interaction, we have calculated NMR spectra, which qualitatively reproduced the observation below H-C but the one above H-C showed discrepancy.

A quasi-one-dimensional quantum spin antiferromagnet Cu3Mo2O9, which possesses the two spin degrees of freedom, the linear chain site and dimer-like site weakly interacting with one another, has recently been reported by permittivity and ESR to show a charge instability in its magnetically ordered state. In this article, we report the study on NMR and magnetization to show the existence of the field-induced magnetic phase transition accompanied by the charge anomaly at 8 T.

We made small superconducting coils and measured the total AC loss under the AC and DC external magnetic fields. The coil winding conditions are that 1, 5, and 10 N of the winding tensions and bobbin materials are glass fiber reinforced plastic (GFRP) and Dyneema fiber reinforced plastic (DFRP). The GFRP and the DFRP have thermal strain properties of contraction and expansion respectively with cooldown to cryogenic temperature. Mechanical losses were estimated from the measured data. According to the estimated mechanical losses, the loss of the coil whose bobbin material is the DFRP was small because the DFRP bobbin expanded and hence the winding was tightly fixed. Approximate 10 to 13% of the AC loss can be reduced by only changing the bobbin material from the GFRP to the DFRP. © 2010 IEEE.

Zero- and longitudinal-field muon-spin-relaxation (ZF- and LF-mu SR) measurements were carried out on the impurity-doped spin gap systems TlCu1-xMgxCl3. In a slightly doped case of x = 0.0047, no evidence for a static internal magnetic field was observed down to 20 mK although the specific heat indicated the magnetic phase transition at T = 0.70 K as previously reported [J. Phys. Soc. Jpn. 78, 074705 (2009)]. Above x = 0.006, the existence of a static internal magnetic field is confirmed by LF-mu SR, and with increasing Mg concentration of x, the internal magnetic field, and the volume fraction of a spin frozen region where the static internal magnetic field appears increase simultaneously. These results suggest that the magnetic ground state changes from the spin singlet state to a spin fluctuating state, and to a spin frozen state by the impurity doping, and also suggest there exist a threshold doping ratio in the appearance of a static staggered moment in the S = 1/2 dimer systems weakly coupled by three-dimensional interactions.

In order to investigate the local structure in the CuO2 plane of the La-based lhigh-T-c cuprate La2-xBaxCuO4, we have carried out measurements of Cu-63-NMR on single crystals with x=0.08 and 0.10 at low temperatures down to 4 K. In both samples, the local structure deduced from observed NMR spectra agreed with the averaged structure. In high-temperature region around 300K, detailed investigation of NMR line width has shown the existence of the local structure different from the averaged structure; that is, CuO6 octahedra tilt from the c-axis toward the random direction by approximately 1.4 deg.

S = 1/2 one-dimensional antiferromagnet KCuGaF6, which has the alternation of both the g-tensor and Dyzaloshinsky-Moriya (DM) vector along the chain, is theoretically expected to show the field-induced spingap. We have measured the longitudinal nuclear spin relaxation rate T-1(-1) of F-19-NMR and found that it shows the thermal activation type temperature dependence, indicating the existing of the spingap. The field-dependence of the gap is found to be proportional to H-2/3. These results are consistent with the theory of Affleck and Oshikawa.

We measured mechanical loss in an AC magnetic field for a short Bi-2223 superconducting tape. Experimental results indicated that mechanical loss decreased as tension applied to the sample tape increased. Furthermore, when a sample holder with a negative coefficient of thermal expansion was used, mechanical loss reduced. The origin of mechanical loss was confirmed to be the vibration of the sample tape. The expansion holder was the most effective means to reduce the vibration. © 2006 IEEE.

Aluminum nitride (AlN) has been widely used as a heat sink material in conduction-cooled superconducting coils but is hard to process. We have therefore developed an easily processed Dyneema-fiber-reinforced plastic (DFRP) with high thermal conductivity. Making small superconducting coils with DFRP or AlN bobbins, cooling them to a cryogenic temperature by using a refrigerator, and comparing their voltage profiles when DC currents were applied to them, we found the heat-sink effect of DFRP to be almost same as that of AlN because DFRP has a high thermal conductivity and expands when cooled. This expansion increases the contact force between superconducting windings and the DFRP bobbin and thereby improves the transfer of heat from the winding to the DFRP. We think DFRP will be the next-generation heat-sink material. © 2006 IEEE.

We carried out zero-field muon-spin-rotation (ZF-mu SR) measurements in hydrostatic pressures in impurity-doped quantum spin gap system Tl(Cu0.985Mg0.015)Cl-3 to investigate microscopic magnetic properties of the pressure-induced phase. The spontaneous muon spin precession, which indicates the existence of a long-range coherent order, is observed in pressures. With decreasing temperature in 3.1 kbar, the internal static magnetic field at the muon sites H-int tends to saturate to 280 Oe around 4 K, however, decreases to 240 Oe at 2.3 K. These results suggest a rearrangement of ordered spins, and we speculate that the oblique antiferromagnetic phase, which is observed in the pressure of 14 kbar on the pure TlCuCl3, appears in the Mg-doped system in lower pressures.

A quasi-one-dimensional spin system Cu3Mo2O9, which has two spin degrees of freedom, the antiferromagnetic linear chain site and dimer-like site weakly interacting with each other, shows a 3D-antiferromagnetic order at T-N=7.9 K. We report the result of Cu-NMR on the single crystal of this system both in antiferromagnetic and paramagnetic states. In field-swept spectra, we observe two types of signals, which come from the chain site and the dimer-like site, respectively. With increasing resonant frequency, the signal from chain site shifts to lower magnetic field, while that from dimer-like site to higher field. This anomalous behaviour of the chain-site signal is explained in terms of the cancellation between the hyperfine field of about 8.5 T at the chain-site Cu and the applied field. A weak but finite hyperfine field of the dimer-like site indicates that there exists a localized spin on the site, indicating that it is not a singlet state.

Muon-spin-rotation (mu SR) measurements under hydrostatic pressures in Tl(Cu(1-x)Mg(x))Cl(3) with x = 0.015 were carried out in order to investigate the reported crossover from impurity-induced phase to pressure-induced phase. The spontaneous muon-spin-rotation, which indicates the existence of a long-range coherent order, is observed under pressures in zero field. With decreasing temperature in 3.1 kbar, the internal static magnetic field H(int) saturates to 280 gauss, however, with increasing pressure at 2.3 K, H(int) saturates to 320 gauss. This discrepancy suggests a new phase inside the ordered phase in the phase diagram.

We carried out zero-field muon-spin-rotation (ZF-μSR) measurements in hydrostatic pressures in impurity-doped quantum spin gap system Tl(Cu 0.985Mg0.015)Cl3 to investigate microscopic magnetic properties of the pressure-induced phase. The spontaneous muon spin precession, which indicates the existence of a long-range coherent order, is observed in pressures. With decreasing temperature in 3.1 kbar, the internal static magnetic field at the muon sites Hint tends to saturate to 280 Oe around 4 K, however, decreases to 240 Oe at 2.3 K. These results suggest a rearrangement of ordered spins, and we speculate that the oblique antiferromagnetic phase, which is observed in the pressure of 14 kbar on the pure TlCuCl3, appears in the Mg-doped system in lower pressures. © 2010 IOP Publishing Ltd.

Zero- and longitudinal-field muon spin relaxation (ZF- and LF-mu SR) measurements were carried out in pressures up to 2 kbar in impurity-doped TlCu0.985Mg0.015Cl3 of which the magnetic phase-transition temperature under ambient pressure in zero field is 2.85 K. The spontaneous muon spin precession, which indicates the existence of a long-range coherent order, is observed above 0.5 kbar at 2.3 K. In ambient pressure, the existence of the static internal magnetic field is confirmed by LF-mu SR measurements, however, no rotational signal is observed down to 0.29 K. These results indicate that the crossover reported by IImamura [Phys. Rev. B 74, 064423 (2006)] is the continuous change from a short-range order to a long-range coherent order.

Zero- and longitudinal-field muon-spin-relaxation (ZF- and LF-mu SR) measurements were carried out down to T = 20 mK on the slightly doped quantum spin system TlCu1-xMgxCl3 with x = 0.0047, in which a magnetic phase transition is observed at 0.70 K by specific heat measurement. LF-mu SR measurements above and below the phase transition temperature revealed that the impurity-induced magnetic moments of the Cu-3d spins slowly fluctuate at a frequency of 0.74 MHz at T = 20 mK, far below the magnetic phase transition temperature. It is possible that the impurity-induced magnetic moments are in an exotic state where they fluctuate while preserving their wave vector and relative phase, i.e., a coherently fluctuating state.

We have studied the vibration of superconducting windings during AC excitation of coils. Our previous study revealed that mechanical loss due to the vibration of a coil that had an expansion bobbin during cool-down was smaller than that of a coil having a contraction bobbin. However, critical currents of the coil with the expansion bobbin decreased as the number of thermal cycles increased between room and cryogenic temperatures. Hence, excessive expansion of the bobbins causes degradation of coil performance. In this study, we fabricated superconducting coils whose bobbins expanded a little, and measured the mechanical losses of those coils. We present here the expansion properties of the bobbins, test results of the critical currents and mechanical losses, and the dependence of the thermal cycles on those results. © 2009 IEEE.

The spin-lattice relaxation rate T-1(-1) of H-1-NMR has been measured in (CH3)(2)CHNH3Cu(ClxBr1-x)(3) with x = 0 and 0.35, in order to investigate their microscopic magnetism. Previous macroscopic magnetization and specific heat measurements suggested that these two exist in a singlet-dimer phase. The temperature dependence of T-1(-1) in an x = 0 system decreased exponentially toward zero, providing microscopic evidence of a gapped singlet ground state, which is consistent with results of macroscopic experiments. At the same time, in the x = 0.35 system, T-1(-1) showed a sharp peak structure at approximately 7.5 K but no splitting of H-1-NMR spectra, indicative of magnetic ordering. We discuss the observed sharp peak structure in the x = 0.35 system with the soft mode toward the exotic magnetic ground state suggested by recent mu SR experiments.

C 63/65 u - and C 35/37 l-NMR experiments were performed to investigate triplet localization in the S=1/2 dimer compound NH4 CuCl3, which shows magnetization plateaus at one-quarter and three-quarters of the saturation magnetization. In C 63/65 u-NMR experiments, signal from only the singlet Cu site was observed, because that from the triplet Cu site was invisible due to the strong spin fluctuation of onsite 3d spins. We found that the temperature dependence of the shift of C 63/65 u-NMR spectra at the singlet Cu site deviated from that of macroscopic magnetization below T=6K. This deviation is interpreted as the triplet localization in this system. From the C 35/37 l-NMR experiments at the 1/4-plateau phase, we found the two different temperature dependences of Cl shift, namely, the temperature dependence of one deviates below T=6K from that of the macroscopic magnetization as observed in the C 63/65 u-NMR experiments, whereas the other corresponds well with that of the macroscopic magnetization in the entire experimental temperature region. We interpreted these dependences as reflecting the transferred hyperfine field at the Cl site located at a singlet site and at a triplet site, respectively. This result also indicates that the triplets are localized at low temperatures. C 63/65 u-NMR experiments performed at high magnetic fields between the one-quarter and three-quarters magnetization plateaus have revealed that the two differently oriented dimers in the unit cell are equally occupied by triplets, the fact of which limits the theoretical model on the periodic structure of the localized triplets. © 2009 The American Physical Society.

Muon-spin-relaxation (mu SR) measurements were carried out on the randomness-introduced quantum spin system Tl1-xKxCuCl3. Zero-field mu SR measurement results reveal that the rapid increase of the mu SR rate lambda with decreasing temperature in the case of x = 0.58. From the result of longitudinal-field mu SR measurements, a finite relaxation remains at 4000 G, which means that Cu-3d spins fluctuate down to 20 mK although the slowing down of the frequency of the spin fluctuation occurs. These results suggest that there is a quite possibility of the existence of an ordered or spin frozen phase in a neighboring concentration of x around 0.58. (C) 2008 Elsevier B.V. All rights reserved.

(CH3)(2)CHNH3CuCl3 is one of the best laboratory models for the S = 1/2 spin ladder comprising ferromagnetic rungs and antiferromagnetic legs. We have determined the phase boundary between the low-field spin liquid and the field-induced ordered phase of this compound to 100 mK in temperature by means of specific-heat and magnetocaloric-effect measurements for two crystal orientations, with the magnetic field perpendicular to either the so-called B or C plane. The critical field of antiferromagnetic ordering shows power-law dependences on temperature, indicative of the Bose-Einstein condensation of spin triplets due to closing of the energy gap by the field.