Superconducting Properties of RuSr₂RECu₂O₈ (RE = Eu, Gd)

The superconducting properties of the magnetic (TM = 130 K) superconductors RuSr2RECu2O8 (RE = Eu, Gd) (Tc;Eu = 27.5 K, Tc;Gd = 46 K) were investigated using resistance and ac-susceptibility measurements. Tc;Eu showed a magnetic-¯eld dependence up to 14 T which can be described by the empirical relation Tc(B) = Tc(0)[1-B/Bc(0)]1/2, contrary to previous reports pointing out the possibility of phase separation in the ruthenocuprates. A plausible explanation of this discrepancy is based on the signi¯cance of the Ru5+/Ru4+ ratio in the ruthenocuprates a®ecting the competition between antiferromagnetic superexchange and ferromagnetic double exchange in these compounds.