Magnetic and superconducting properties of RuSr2GdCu2O8

Magnetization measurements of the magnetic (T-M approximate to 138 K) superconductor (T-S approximate to 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 mu(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 Ru5+ and Ru4+ ions and antiferromagnetic superexchange involving Ru5+ ions. This causes magnetic-phase separation in ferromagnetic (non-superconducting) and anti ferromagnetic (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. (C) 2007 Elsevier B.V. All rights reserved.