Ferromagnetic correlations and mixed Ru valence in the magnetic superconductor RuSr2(Eu, Gd)Cu2O8 -: art. no. 054442

We present a characterization through magnetization measurements as a function of field and temperature of the magnetic superconductor RuSr2RCu2O8 (R = Eu or Gd). The effective magnetic moment was carefully determined from measurements up to 900 K. For both compounds we have obtained almost the same value, mu (eff)similar to3.15(4)mu (beta). This value confirms recent x-ray absorption experiments where a mixed valence state (40% Ru4+-60% Ru5+) was proposed. We have found that the predominant interaction among Ru ions is ferromagnetic [Theta = 100(3) K and Theta = 87(3) K for the Eu and Gd compounds, respectively], but the high-field susceptibility curve displays the characteristics of an antiferromagnetic transition. The possible type of order of the Ru moments is discussed in view of these and previous experiments. We propose a type-I ordering (antiferromagnetically coupled ferromagnetic RuO2 planes) and give reasons supporting this choice.