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Muon spin rotation measurements of Sr 2 Y(Ru 1Ϫu Cu u )O 6 ͑for uϭ0.1) reveal two distinct muon sites: one located in a SrO layer ͑which is superconducting at low temperatures͒ and the other in a Y(Ru 1Ϫu Cu u )O 4 layer ͑which is magnetically ordered at low temperatures͒. A precursor spin-glass state due to the Ru moments is detected in high fields ͑Ϸ3.3 kOe͒ in Y(Ru 1Ϫu Cu u )O 4 layers, with a spin-glass temperature of T G ϭ29.25 K. The Y(Ru 1Ϫu Cu u )O 4 layers order ferromagnetically in the a-b planes at the Néel temperature, T N Ϸ23 K. This in-plane ferromagnetism alternates direction between adjacent Y(Ru 1Ϫu Cu u )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,onset Ϸ49 K, this superconductivity is adversely affected by the Ru moments that fluctuate for TϾT 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 TϽT G . In fact, resistance measurements only show zero resistance below T N , at which temperatures the Ru moments that fluctuated for TϾT 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.

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