Haruo Niki

Proton NMR in hydrogen-doped superconductor YBa2Cu3O7-δH0.2

Abstract Proton NMR in YBa 2 Cu 3 O 7-δ H 0.2 was performed between 77 K and 300 K. The hydrogen atom is diffusing or moving in the crystal above 170 K. Below 150 K it is trapped in the O(1) or O(2) oxygen sites in CuO chain. In the superconducting state below T c of 92 K, the penetration depth of the magnetic field at T = 0 K was determined to be 2500 A.

Magnetotransport in Y1-xGdxCo2 pseudobinary alloys

Thermopower (S), resistivity (ρ), and AC susceptibility of Y 1 xGdxCo2 alloys (0 ≤ x ≤ 1) have been measured at temperatures of 2K-300 K in magnetic field from 0 to 15 T. There are 4 composition regions on the phase diagram of Y 1 x Gd x Co 2 : Kondo-type behavior is observed at 0 < x ≤0.05, magnetic cluster glass state is found for x from about 0.07 to 0.12. At higher Gd content, x≥0.13 the ground state of the 4f localized moment subsystem is ferromagnetic. The magnetoresistivity (Δρ) shows unusual large positive values in the magnetically ordered phase 0.15 < x ≤0.3.

Proton nuclear spin-lattice relaxation in hydrogen-doped superconductor YBa2Cu3O6.94H0.2

Nuclear spin-lattice relaxation time T1 of 1H in the hydrogen-doped superconductor YBa2Cu3O6.94H0.2 with Tc the spin-lattice relaxation rate T−1 between 77 and 300 K. Just below Tc the spin-lattice relaxation rate T−1 shows a fairly large enhancement at 0.93 Tc, followed by a sharp decrease with decreasing temperature. The hydrogen atoms are trapped in the single atomic site below 150 K and moving in the crystal above 170 K. From the analysis of T1, it is concluded that paramagnetic impurities exist.

Magnetic Field Evolution of a Novel Phase Transition in CeOs4Sb12: 121Sb NMR Study

We report on the evolution of a novel phase transition under magnetic field ( H ) in the filled skutterudite compound CeOs 4 Sb 12 probed by 121 Sb NMR. Measurements of nuclear spin–lattice relaxation rate 121 (1/ T 1 ) for the oriented powder with H ∥[100] have revealed that antiferromagnetic (AFM) spin fluctuations due to the proximity of a quantum critical point at H =0 are markedly suppressed by the application of H , which is consistent with the result indicating that the AFM order at T N =0.9 K with a very small moment of 0.07µ B collapses, exceeding H =1 T. As H increases further, a large reduction in 1/ T 1 T was observed below the temperature T 0 , which varies as a dome against H with a maximum of 1.8 K at H ∼6 T. The values of T 1 T =const. well below T 0 decrease with increasing H , indicating that the residual density of states depends on H . It is suggested that the field-induced novel transition is associated with the freezing of multipole degree of freedom.

Proton nuclear magnetic resonance studies of hydrogen diffusion and electron tunneling in Ni-Nb-Zr-H glassy alloys

Using the Fourier transform of the echo envelope, the proton line shapes, spin-lattice relaxation time, and spin-spin relaxation time have been measured in a (Ni0.36Nb0.24Zr0.40)90H10 glassy alloy at 1.83 T (∼78 MHz) and at temperatures between 1.8 and 300 K. First, the spectral line width decreases abruptly between 1.8 and 2.1 K. Next, it remains almost constant at 13 kHz up to ∼150 K. Finally, the line width decreases as the temperature increases from ∼150 to 300 K. The initial decrease in the spectral line width is ascribed to the distribution of the external field, which is caused by the penetration of vortices in the superconducting state. The subsequent leveling off in the spectral line width is ascribed to the dipole-dipole interaction between protons when hydrogen atoms are trapped into vacancies among the Zr-centered icosahedral Zr5Ni5Nb3 clusters. The final decrease in the spectral line width is ascribed to the motional narrowing of the width that is caused by the movement of hydrogen atoms. The...

59Co NMR in ErCo3

Abstract NMR measurements of 59 Co in ferrimagnetic ErCo3 have been carried out between 4.2 and 120 K in external fields up to 7 T . The zero-field powder spectrum at 4.2 K shows three resolved lines at 23, 65, and 96 MHz . In a field along the c-axis the two lower frequencies decrease with increasing field at 10.01 MHz/T , while the upper one increases at the same rate. The temperature dependence of the upper line gives strong indication that the Co-moment of this sublattice decreases discontinuously at the transition temperature 100 K .

Crystal Structure and Phase Transition of 4-Aminopyridinium Tetrabromoantimonate(III) as Studied by Bromine and Antimony NQR, Proton NMR, and Single Crystal X-Ray Diffraction

The crystal structure of the room temperature phase (RTP) of the title compound was determined at 297 K (monoclinic, space group C2/c, a = 1384.2(2), b = 1377.8(3), c = 755.5(2) pm, β = 121.58(1)°). A complicated disorder was found for the cation. A phase transition from the low-temperature phase (LTP) to the RTP was found at (224 ±1) K (Tc). The 1H NMR spectra showed a sharp motional narrowing at ca. T=Tc , indicating the occurrence of a reorientational motion of the cation in the RTP in accord with the disorder. It was found that another reorientational motion is excited in the LTP. Four 81Br NQR lines (132.71, 115.38, 61.54 and 59.31 MHz at 77 K) and two Sb NQR lines (53.78 and 33.76 MHz at 77 K) were found in the LTP, while a single 81Br NQR line was observed at T> 276 K (ca. 121.80 MHz at 300 K). Crystal dynamics are discussed on the basis of the temperature dependence of the NQR, 1H NMR line width, and 1H NMR T1.

Proton NMR studies on hydrogen-doped high-Tc superconductor

Proton NMR in YBa 2 Cu 3 O 6.94 H x ( x = 0.2 and 0.5) was carried out in order to obtain information about the behavior of the hydrogen and mechanism of the superconductivity. The hydrogen atoms are moving above 200 K but trapped in the crystal sites below 130 K. Below T c , T 1 −1 shows an enhancement and sharp decrease with decreasing temperature.

Localization of Co–3d electrons in Y(Co1−xAlx)2 paramagnetic alloys with itinerant spin fluctuations

Abstract Thermoelectric power (S) and electrical resistivity (ρ) of Y(Co1−xAlx)2 alloys with x in the range of 0–0.15 have been measured at temperatures from 2 to 300 K, in the longitudinal magnetic field of 0–15 T. The present results show that there is a rapid variation of the transport properties and a peculiarity in the dependency of lattice parameter on x at x≈0.03; however, no change of the lattice symmetry has been detected. Although the precise mechanism of the anomaly at x≈0.03 has not been understood, the results suggest that it is related to a change in the localization of the Co–3d electrons caused by the increasing Co–Co separation and by the disorder within the Co-sublattice in the Y(Co1−xAlx)2 alloys.

Thermopower of Fe2P in magnetic fields up to 15 T

Abstract Temperature dependencies of thermopower S ( T ) of Fe 2 P compound with hexagonal structure have been measured between 2 and 300 K in magnetic fields up to 15 T. S ( T ) takes a minimum around 20 K and shows a kink at T c . T c has a large magnetic field H dependence. The S ( T ) curve after cooling from room temperature to 2 K in magnetic field of 15 T reveals two characteristic features, the shoulder between 60 and 70 K and the discontinuity at T c .