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 funct ion of field and temperature of the magnetic superconductor RuSr2RCu2O8 (R = Eu or Gd). The effective magnetic moment was carefully determined from me asurements 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-ra y absorption experiments where a mixed valence state (40% Ru4+-60% Ru5+) wa s 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 comp ounds, respectively], but the high-field susceptibility curve displays the characteristics of an antiferromagnetic transition. The possible type of or der of the Ru moments is discussed in view of these and previous experiment s. We propose a type-I ordering (antiferromagnetically coupled ferromagneti c RuO2 planes) and give reasons supporting this choice.