Magnetization of superconducting RuSr 2 GdCu 2 O 8 and of concurrent phases H.F. Braun a, * , T.P. Papageorgiou a , T. Herrmannsd€ orfer b , L. Bauernfeind a , O. Korf a a Physikalisches Institut, Universit€ at Bayreuth, D-95440 Bayreuth, Germany b Forschungszentrum Rossendorf (FZR), D-01314 Dresden, Germany Abstract The appearance of impurity phases belonging to the quaternary oxide system SrO–GdO 1:5 –CuO–RuO 2 in Ru-1212 samples is difficult to avoid. While the formation of the ferromagnetic compound SrRuO 3 may be suppressed by a precursor route involving Sr 2 GdRuO 6 , this latter compound shows magnetization anomalies below 30 K. We present evidence that magnetization anomalies observed for some Ru-1212 samples in this temperature range are not due to trace amounts of Sr 2 GdRuO 6 . Rather, the effect appears to be related to the establishment of inter-granular super- conductivity and to the vortex dynamics in Ru-1212. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Superconductivity; DC magnetization; Ruthenocuprates; Ruthenates 1. Introduction The layered ruthenocuprates RuSr 2 LnCu 2 O 8 (Ru-1212 structure) and RuSr 2 (Ln 1þx Ce 1x )Cu 2 O 10 (Ru-1222 structure; Ln ¼ lanthanide or Y for both structures) consist of pairs of CuO 2 planes alter- nating with perovskite-like sheets of vertex sharing RuO 6 octahedra. First reported in 1995 [1,2], they appear today to provide the first example of su- perconductivity [3] developing at temperatures far below the transition into a weakly ferromagnetic state, with coexistence of both ordering pheno- mena [4–7]. The weak ferromagnetism is associ- ated with the ordered antiferromagnetic state of the Ru moments which appears at about 138 K in Ru-1212, while the Gd moments order antiferro- magnetically below 3 K [8–10]. However, similar perovskite-like sheets are also the characteristic structural feature of some strontium ruthenates. The presence of perovskite impurities such as SrRuO 3 , with a known tran- sition to ferromagnetism at 160 K [11], and an apparent dependence of superconducting and magnetic properties on sample preparation con- ditions made it difficult to attribute the ordering phenomena to the Ru-1212 and Ru-1222 phases. Improved sample preparation, involving a treat- ment step in inert atmosphere which results in the * Corresponding author. Tel.: +49-921-553330; fax: +49-921- 553647. E-mail address: hans.braun@uni-bayreuth.de (H.F. Braun). 0921-4534/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(03)00636-1 www.elsevier.com/locate/physc Physica C 387 (2003) 26–32