Magnetic and superconducting properties of RuSr₂GdCu₂O₈

Magnetization measurements of the magnetic (T_M ≈ 138 K) superconductor (T_S ≈ 42 K) RuSr₂GdCu₂O₈ in pulsed magnetic fields up to 47 T reveal at 48 K and the highest available magnetic field a Ru contribution to the measured magnetic moment of about 2.2 µ_B/ formula unit. This value is indicative of a mixed-valence state of the Ru ions involving Ru⁵⁺(S = 3/2) and Ru⁴+(S = 1) ions. We propose that the mixed-valence state combined with charge transport in the RuO₂ planes results in a competition between ferromagnetic double exchange involving Ru>sup>5+ and Ru⁴⁺ ions and antiferromagnetic superexchange involving Ru⁵⁺ ions. This causes magnetic-phase separation in ferromagnetic (non-superconducting) and antiferromagnetic (superconducting) domains. We conclude that the magnetic and superconducting properties of the ruthenocuprates critically depend on the Ru⁵⁺/Ru⁴⁺ ratio which can be affected by the preparation conditions.