Spin-glass behavior, spin fluctuations, and superconductivity inSr2Y(Ru1−uCuu)O6

Abstract

Muon spin rotation measurements of ${\mathrm{Sr}}_{2}\mathrm{Y}({\mathrm{Ru}}_{1\ensuremath{-}u}{\mathrm{Cu}}_{u}){\mathrm{O}}_{6}$ (for $u=0.1)$ reveal two distinct muon sites: one located in a SrO layer (which is superconducting at low temperatures) and the other in a $\mathrm{Y}({\mathrm{Ru}}_{1\ensuremath{-}u}{\mathrm{Cu}}_{u}){\mathrm{O}}_{4}$ layer (which is magnetically ordered at low temperatures). A precursor spin-glass state due to the Ru moments is detected in high fields (\ensuremath{\approx}3.3 kOe) in $\mathrm{Y}({\mathrm{Ru}}_{1\ensuremath{-}u}{\mathrm{Cu}}_{u}){\mathrm{O}}_{4}$ layers, with a spin-glass temperature of ${T}_{G}=29.25\mathrm{K}.$ The $\mathrm{Y}({\mathrm{Ru}}_{1\ensuremath{-}u}{\mathrm{Cu}}_{u}){\mathrm{O}}_{4}$ layers order ferromagnetically in the a-b planes at the N\'eel temperature, ${T}_{N}\ensuremath{\approx}23\mathrm{K}.$ This in-plane ferromagnetism alternates direction between adjacent $\mathrm{Y}({\mathrm{Ru}}_{1\ensuremath{-}u}{\mathrm{Cu}}_{u}){\mathrm{O}}_{4}$ planes, resulting in a net antiferromagnetic structure. Although the onset of superconductivity is observed both by electron spin resonance and by dc susceptibility to occur for temperatures up to about ${T}_{c,\mathrm{onset}}\ensuremath{\approx}49\mathrm{K},$ this superconductivity is adversely affected by the Ru moments that fluctuate for $T>{T}_{N}$ producing magnetic fields that break pairs in the SrO layers. The muons, as well as other probes, sense the more-robust static superconductivity for $T<{T}_{G}.$ In fact, resistance measurements only show zero resistance below ${T}_{N},$ at which temperatures the Ru moments that fluctuated for $T>{T}_{N}$ are frozen in-plane. Hence strictly speaking, the superconducting transition temperature is the same as ${T}_{N},$ which is far below ${T}_{c,\mathrm{onset}}.$ Below ${T}_{N}$ there are no pair breaking fluctuating magnetic fields in the SrO layers where the hole condensate resides.