Superconductivity and magnetism in Sr2Y(Ru1-uCuu)O-6 and in Ba2Gd(Ru1-uCuu)O-6

Muon spin rotation and electron spin resonance data on sintered samples of superconducting Sr2Y(Ru1-uCuu)O-6 and non-superconducting Ba2Gd(Ru1-uCuu)O- 6 are reported, both for u = 0.1. In the case of Sr2Y(Ru1-uCuu)O-6, the SrO layers are found to be p-type and to exhibit an onset for superconductivit y at approximate to 45 K - a temperature considerably lower than the spin-o rdering temperature of the Cu ions (approximate to 86 K), indicating that t he Cu ions themselves do not play a significant role in the superconductivi ty. Below T-c, the fluctuating Ru moments begin to slow down and freeze, so that at about approximate to 29.3 K a spin-glass state is observed, which gives way to ferromagnetic ordering of the Ru ions in the Y(Ru1-uCuu)O-4 pl anes, with the magnetization alternating direction in the a-b plane from on e magnetic layer to the next. These data confirm our earlier discovery that fluctuating moments (in this case, Ru moments) interfere with pairing. Ba2 Gd(Ru1-uCuu)O-6 shows no evidence of superconductivity. which we interpret as due to pair breaking by the L = 0 magnetic Gd ions, which are not crysta l-field split. (C) 2001 Elsevier Science B.V. All rights reserved.