The Unusual Phase-Transitions in Ruthenate–Cuprate Superconducting Ferromagnets

We have systematically investigated the magnetic, electrical, and structural properties of the high-purity superconducting ferromagnet RuSr2GdCu2O8, in which superconductivity and significant ferromagnetism have been reported to coexist. Our results show that there may exist in this compound a novel superconducting state which is characterized by the absence of a bulk Meissner effect, the presence of negligible superconducting condensation energy, and the appearance of an unusually large effective penetration depth. Such a state appears to have a nonuniform filamentary structure existing in the less magnetic walls between the ferromagnetic domains or a fine-grain structure beset by ferromagnetic walls between antiferromagnetic “domains,” similar to the case of NdNi2B2C. These results are presented and contrasted with the recently reported Meissner effect in RuSr2GdCu2O8 in terms of the various phase transition sequences predicted.