The unusual phase-transitions in ruthenate-cuprate superconducting ferromagnets

We have systematically investigated the magnetic, electrical, and structura l properties of the high-purity superconducting ferromagnet RuSr2GdCu2Os8 i n 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 Meis sner effect, the presence of negligible superconducting condensation energy , and the appearance of an unusually large effective penetration depth. Suc h a state appears to have a nonuniform filamentary structure existing in th e less magnetic walls between the ferromagnetic domains or a fine-grain str ucture beset by ferromagnetic walls between antiferromagnetic "domains," si milar to the case of NdNi2B2C. These results are presented and contrasted w ith the recently reported Meissner effect in RuSr2GdCu2O8 in terms of the v arious phase transition sequences predicted.