Superconducting properties of RuSr2RECu2O8RuSr2RECu2O8(RE=Eu,Gd)(RE=Eu,Gd)

The superconducting properties of the magnetic (TM≈130K) superconductors RuSr2RECu2O8RuSr2RECu2O8(RE=Eu,Gd)(RE=Eu,Gd) (Tc,Eu≈27.5K, Tc,Gd≈46K) were investigated using resistance and AC-susceptibility measurements. Tc,EuTc,Eu showed a magnetic-field dependence up to 14T which can be described by the empirical relation Tc(B)=Tc(0)[1-B/Bc(0)]1/2Tc(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 significance of the Ru5+/Ru4+Ru5+/Ru4+ ratio in the ruthenocuprates affecting the competition between antiferromagnetic superexchange and ferromagnetic double exchange in these compounds.