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 RuSr ₂GdCu₂O₈, 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 NdNi ₂B₂C. These results are presented and contrasted with the recently reported Meissner effect in RuSr₂GdCu₂O ₈ in terms of the various phase transition sequences predicted.