Spin-glass behavior, spin fluctuations, and superconductivity in Sr₂Y(Ru_1-uCu_u)O₆

Muon spin rotation measurements of Sr₂Y(Ru_1-uCu_u)O₆ (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 Y(Ru_1-uCu_u)O₄ layer (which is magnetically ordered at low temperatures). A precursor spin-glass state due to the Ru moments is detected in high fields (≈3.3 kOe) in Y(Ru_1-uCu_u)O₄ layers, with a spin-glass temperature of T_G =29.25 K. The Y(Ru_1-uCu_u)O₄ layers order ferromagnetically in the a-b planes at the Néel temperature, T_N≈23 K. This in-plane ferromagnetism alternates direction between adjacent Y(Ru_1-uCu_u)O₄ 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.onset≈49 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 TG. 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.onset. Below T_N there are no pair breaking fluctuating magnetic fields in the SrO layers where the hole condensate resides.