Spin fluctuations in the ruthenium oxidesRuO2,SrRuO3,CaRuO3,andSr2RuO4probed by Ru NMR

Abstract

By means of Ru NMR, magnetic properties in three-dimensional (3D) perovskites ${\mathrm{SrRuO}}_{3}$ (ferromagnet) and ${\mathrm{CaRuO}}_{3}$ (exchange enhanced paramagnet) were investigated and compared with those in the two-dimensional (2D) layered perovskite ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ (superconductor), ${\mathrm{RuO}}_{2},$ and Ru metal (Pauli paramagnet). We found that $4d$-spin contributions in the Knight shift and ${}^{101}{(1/T}_{1})$ of Ru are predominant in ${\mathrm{SrRuO}}_{3},$ ${\mathrm{CaRuO}}_{3},$ and ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4},$ but not in nonmagnetic ${\mathrm{RuO}}_{2}$ and Ru metal. The experimental results that a Stoner factor for ${\mathrm{CaRuO}}_{3}$ is close to 1 and that a correlation factor estimated from modified-Korringa behavior of $\mathrm{Ru}\ensuremath{-}{T}_{1}$ is much less than 1 indicate that ${\mathrm{CaRuO}}_{3}$ is a nearly ferromagnetic (FM) metal dominated by low-frequency and long-wavelength components of the spin fluctuations (SF). Even though the Stoner factor for ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ also indicates the closeness to the ferromagnetism, the SF in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ are different from those in ${\mathrm{CaRuO}}_{3}.$ The results combined with the previous ${}^{17}\mathrm{O}$ NMR study in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ indicate that the in-plane low-frequency components of SF are exchange enhanced without any significant q dependence due to 2D electronic character (2D nearly FM), while the out-of-plane low-frequency component of SF shows the existence of antiferromagnetic (AFM) SF between layers at low temperature. We propose that this evolution from 3D to 2D nearly FM SF is relevant to the onset of spin-triplet superconductivity in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}.$ The 2D nearly FM SF in the in-plane ${4d}_{\mathrm{xy}}\ensuremath{-}{p}_{\mathrm{\ensuremath{\pi}}\mathrm{}}$ band may play a significant role for the stabilization of parallel spin pairing state within the basal plane among various representations of spin-triplet order parameter in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}.$