Magnetization of RuSr₂GdCu₂O₈ in pulsed magnetic fields up to 47 T

Magnetization measurements in pulsed magnetic fields up to 47 T were utilized to estimate the average Ru moment in polycrystalline RuSr₂GdCu₂O₈. A NbSr₂GdCu₂O₈ sample was used as reference. The extracted Ru moment of about 1.8µ_B at 4.2 K and 47 T is indicative of a mixed-valence state of Ru involving Ru⁵⁺ and Ru⁴⁺ ions with magnetic moments of 2µ_B/Ru⁵⁺ and 0.9µ_B/Ru⁴⁺. The estimated ratio of Ru⁵⁺:Ru⁴⁺≈ 87%:13% corresponds to a hole concentration p≈0.065 in the CuO₂ planes suggesting an underdoped nature of the superconducting state. We propose that the magnetic structure of the Ru moments corresponds to an antiferromagnetic phase, involving only Ru⁵⁺ ions, interrupted by ferromagnetic stripes, where charge transfer between Ru⁴⁺and Ru⁵⁺ ions takes place.