深澤 英人

dc (χ) and ac (χac) magnetic susceptibilities, field dependence of isothermal magnetization and specific heat Cpmeasurements have been performed on a pyrochlore geometrically frustrated niobate Tb2Nb2O7. The χ characteristics is consistent with collective paramagnetism down to at least T ≈ 2 K with effective nearest-neighbor antiferromagnetic interactions, Jeff= -1.5 K, among magnetic ions. Crystal-field calculation shows that Tb3+ground state is an isolated Ising-like S = 1/2 doublet with anisotropic g tensor and Tb3+spins pointing along local 〈111〉 directions of tetrahedral unit of pyrochlore structure. The ground-state doublet is further modified by the short-range spin-spin correlations, giving rise to a rounded peak at 1.9 K in zero-field magnetic specific heat Cmag. In addition, the frequency-dependent peak in χacat about 0.8 K indicates spin-glass-like freezing of Tb3+spins in Tb2Nb2O7. Tb2Nb2O7is compared with other geometrically frustrated Tb-based pyrochlores, e.g., a spin-glass Tb2Mo2O7and a collective paramagnet-spin liquid Tb2Ti2O7.

Magnetic properties in the normal state of spin-triplet superconductor Sr2RuO4 are discussed on the basis of Ru- and O-NMR and neutron-diffraction (ND) measurements reported so far. Besides strong incommensurate antiferromagnetic (IC-AFM) fluctuations probed by the ND measurements, it is shown that q-independent fluctuations are present over a wide temperature range up to 500 K from the comparison between NMR and ND results. From an anisotropy in the nuclear spin lattice relaxation rate 1/T-1 at both the Ru and O sites, it was found that the IC-AFM fluctuations are anisotropic so that the out-of-plane component of low-frequency dynamical susceptibility, chi'(IC,out)(q,omega) is by a factor of similar to3 larger than the in-plane one, chi"(IC,in)(q, omega). These unique characters in magnetic fluctuations are implied to have some relevance for the occurrence of spin-triplet superconductivity.

We report Ru-99 Knight-shift (K-99) measurement on the high-quality Sr2RuO4 with a superconducting transition temperature T-c similar to 1.5 K. lts spin component, K-99(s) due to the core polarization effect caused by Ru-4d spins, is nearly one order of magnitude larger than that of the O-17 Knight shift (K-17(s)). The 4d-spin susceptibility that has been more precisely measured from K-99(s) than K-17(s) shows no change across T-c at all. The present result reinforces that Sr2RuO4 is the spin-triplet p-wave superconductor in which electrons are bound together in parallel-spin pairs parallel to the RuO2 plane.