Superconductivity and magnetism in Sr₂Y(Ru_1-uCu_u)O₆ and in Ba₂Gd(Ru_1-uCu_u)O₆

Muon spin rotation and electron spin resonance data on sintered samples of superconducting Sr₂Y(Ru_1-uCu_u)O₆ and non-superconducting Ba₂Gd(Ru_1-uCu_u)O₆ are reported, both for u = 0.1. In the case of Sr₂Y(Ru_1-uCu_u)O₆, the SrO layers are found to be p-type and to exhibit an onset for superconductivity at ≈45 K - a temperature considerably lower than the spin-ordering temperature of the Cu ions (≈86 K), indicating that the Cu ions themselves do not play a significant role in the superconductivity. Below T_c, the fluctuating Ru moments begin to slow down and freeze, so that at about ≈29.3 K a spin-glass state is observed, which gives way to ferromagnetic ordering of the Ru ions in the Y(Ru_1-uCu_u)O₄ planes, with the magnetization alternating direction in the a-b plane from one magnetic layer to the next. These data confirm our earlier discovery that fluctuating moments (in this case, Ru moments) interfere with pairing. Ba₂Gd(Ru_1-uCu_u)O₆ shows no evidence of superconductivity, which we interpret as due to pair breaking by the L = 0 magnetic Gd ions, which are not crystal-field split.