NMR probe of magnetism and superconductivity in ruthenate oxides

We review recent NMR investigations on superconducting and magnetic propert ies in layered perovskite Sr2RuO4, isostructural to a high-ir, cuprate. Mea surements 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 uncovere d pronounced anisotropy in spin-fluctuation spectrum in Sr2RuO4. The in-pla ne low-frequency components of the dynamical susceptibility are exchange-en hanced with a weak q dependence associated with the two-dimensional (2D) ch aracter in electronic structure. By contrast, its out-of-plane component is unexpectedly enhanced by AF spin fluctuations upon cooling below T* simila r to 130 K where the c-axis resistivity shows metallic behavior. We have fo und that the slightly-distorted cubic perovskite CaRuO3, is a nearly ferrom agnetic metal dominated by ferromagnetic spin fluctuations with a Stoner fa ctor alpha = 0.98. Contrasting behavior of spin fluctuations between Sr2RuO 4 and CaRuO3 is due to their different dimensionality in electronic structu re with the closeness to ferromagnetism. We remark that a simple ferromagne tic-spin-fluctuation-mediated mechanism with analogy to He-3 is not appropr iate for an onset of spin-triplet p-wave superconductivity in Sr2RuO4. Alte rnately, we propose possible scenario that the in-plane local exchange inte raction, which originates presumably from the Hund's coupling between the 4 d(xy) and 4d(xy,yz) orbitals, may play a role for the occurrence of spin-tr iplet p-wave superconductivity. (C) 1999 Elsevier Science S.A. All rights r eserved.