Ferromagnetic correlations and mixed Ru valence in the magnetic superconductor RuSr2(Eu,Gd)Cu2O8

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, μ_eff~3.15(4)μ_β. 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 [Θ=100(3) K and Θ=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.