NMR probe of magnetism and superconductivity in ruthenate oxides

We review recent NMR investigations on superconducting and magnetic properties in layered perovskite Sr₂RuO₄, isostructural to a high-T_c cuprate. Measurements of the Knight shift and the nuclear spin-lattice relaxation rate 1/T₁ have identified its Cooper-pair spin state as a triplet and uncovered pronounced anisotropy in spin-fluctuation spectrum in Sr₂RuO₄. 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* to approximately 130 K where the c-axis resistivity shows metallic behavior. We have found that the slightly-distorted cubic perovskite CaRuO₃ is a nearly ferromagnetic metal dominated by ferromagnetic spin fluctuations with a Stoner factor α = 0.98. Contrasting behavior of spin fluctuations between Sr₂RuO₄ and CaRuO₃ 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 Sr₂RuO₄. Alternately, we propose possible scenario that the in-plane local exchange interaction, which originates presumably from the Hund's coupling between the 4d_xy and 4d_{xz, yz} orbitals, may play a role for the occurrence of spin-triplet p-wave superconductivity.