Ru moment in the magnetically ordered superconductor RuSr₂GdCu₂O₈

Magnetization measurements of the superconducting (Tc ≈ 47 K) and magnetically ordered (TRuM ≈ 130 K)RuSr₂GdCu₂O₈ (Ru1212) have been performed in pulsed magnetic fields up to 47 T. The average Ru-moment, determined by using NbSr₂GdCu₂O₈ as reference, is 1.8 μB suggesting that the investigated sample is in a mixed valence state containing 87% Ru⁵⁺ (2 μ_B) and 13% Ru⁴⁺ (0.9 μ_B). This ratio is consistent with an underdoped nature of the superconducting state with a hole concentration in the CuO₂ plane of p ≈ 0.065. It is suggested that the magnetic structure of Ru1212 consists of a main antiferromagnetic phase formed by Ru⁵⁺ions interrupted by ferromagnetic stripes, where double exchange between Ru⁵⁺ and Ru⁴⁺ ions takes place. Different Ru⁵⁺/Ru⁴⁺ ratios, due to different preparation conditions, could explain some of the diverse superconducting and magnetic properties reported in the literature for Ru1212.