Evolution of normal-state magnetic fluctuations by Ca and Ti substitutions inSr2RuO4:87Sr−NMRstudy

Abstract

We report magnetic properties of $({\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{Ca}}_{x}){\mathrm{RuO}}_{4}$ and ${\mathrm{Sr}}_{2}({\mathrm{Ru}}_{1\ensuremath{-}y}{\mathrm{Ti}}_{y}){\mathrm{O}}_{4}$ investigated by ${}^{87}\mathrm{Sr}$ nuclear magnetic resonance NMR to shed light on the effect of multibands on the magnetic character in the spin-triplet superconductor ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}.$ In $({\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{Ca}}_{x}){\mathrm{RuO}}_{4},$ the Knight shift ${(}^{87}K)$ at 1.4 K increases progressively up to $x=1.5.$ The Korringa relation between K and the nuclear-spin-lattice-relaxation rate ${1/T}_{1}$ divided by temperature ${(1/T}_{1}T)$ reveals that ferromagnetic (FM) fluctuations develop with the increasing Ca content. In ${\mathrm{Sr}}_{2}({\mathrm{Ru}}_{1\ensuremath{-}y}{\mathrm{Ti}}_{y}){\mathrm{O}}_{4},$ in contrast, it is found that anisotropic incommensurate (IC) antiferromagnetic (AFM) fluctuations present in the $y=0$ material are enhanced by a small amount of Ti substitution: ${1/T}_{1}T$ is enhanced with the magnetic field parallel to the ${\mathrm{RuO}}_{2}$ plane, whereas it is unchanged for the field along the c axis in 3% Ti-doped ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}.$ We demonstrate that Ca substitution affects the two-dimensional (2D) $\ensuremath{\gamma}$ band by enhancing the FM fluctuations, and that the Ti-substitution affects the 1D $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ bands by enhancing the IC-AFM fluctuations. The unique substitution effects would relate with the multiband character in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}.$