NMR probe of magnetism and superconductivity in ruthenate oxides

Abstract

We review recent NMR investigations on superconducting and magnetic properties in layered perovskite Sr 2RuO 4, isostructural to a high- T c cuprate. Measurements of the Knight shift and the nuclear spin-lattice relaxation rate 1/ T 1 have identified its Cooper-pair spin state as a triplet and uncovered pronounced anisotropy in spin-fluctuation spectrum in Sr 2RuO 4. The in-plane low-frequency components of the dynamical susceptibility are exchange-enhanced with a weak q → dependence associated with the two-dimensional (2D) character in electronic structure. By contrast, its out-of-plane component is unexpectedly enhanced by AF spin fluctuations upon cooling below T*∼130 K where the c-axis resistivity shows metallic behavior. We have found that the slightly-distorted cubic perovskite CaRuO 3 is a nearly ferromagnetic metal dominated by ferromagnetic spin fluctuations with a Stoner factor α=0.98. Contrasting behavior of spin fluctuations between Sr 2RuO 4 and CaRuO 3 is due to their different dimensionality in electronic structure with the closeness to ferromagnetism. We remark that a simple ferromagnetic-spin-fluctuation-mediated mechanism with analogy to 3He is not appropriate for an onset of spin-triplet p-wave superconductivity in Sr 2RuO 4. Alternately, we propose possible scenario that the in-plane local exchange interaction, which originates presumably from the Hund’s coupling between the 4 d xy and 4 d xz, yz orbitals, may play a role for the occurrence of spin-triplet p-wave superconductivity.