High magnetic field study of RuSr₂GdCu₂O₈

Magnetization measurements in pulsed magnetic fields up to 47 T have been performed on the magnetic (TM,Ru ≈ 133 K) superconductor (TS ≈ 46 K) RuSr2GdCu2O8 in an attempt to determine the Ru valency in this compound. The Ru ions are most probably in a mixed valence state, but the ratio of Ru5+(S=3/2)/Ru4+(S = 1) ≈ 60%/40% suggested by NMR investigations is justified only if one considers the theoretical values of 3 μB/Ru5+ and 2 μB/Ru4+. Mixed valency could lead to a competition between ferromagnetic double exchange, evolving Ru4+ and Ru5+ ions, and antiferromagnetic superexchange, evolving Ru5+ ions, leading to magnetic phase separation. We conclude that the magnetic and superconducting properties of the rutheno-cuprates critically depend on the Ru5+/Ru4+ ratio which can be affected by the preparation conditions.