Abstract
The superconducting properties of the magnetic ( T M ≈ 130 K ) superconductors RuSr 2 RECu 2 O 8 ( RE = Eu , Gd ) ( T c , Eu ≈ 27.5 K , T c , Gd ≈ 46 K ) were investigated using resistance and AC-susceptibility measurements. T c , Eu showed a magnetic-field dependence up to 14 T which can be described by the empirical relation T c ( B ) = T c ( 0 ) [ 1 - B / B c ( 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 Ru 5 + / Ru 4 + ratio in the ruthenocuprates affecting the competition between antiferromagnetic superexchange and ferromagnetic double exchange in these compounds.
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