Coexisting ferromagnetism and superconductivity in hybrid rutheno-cuprate superconductors

We report the observation of microscopically coexisting ferromagnetism and superconductivity in a hybrid ruthenocuprate RuSr/sub 2/GdCu/sub 2/O/sub 8/, with T/sub c//spl les/40 K. By means of various substituents it is established that the superconductivity originates in the CuO/sub 2/ planes and the ferromagnetism in the RuO/sub 2/ planes, as expected. Muon spin relaxation measurements show that the ferromagnetism, with Curie temperature T/sub M/=132 K, is microscopically uniform and originates from the entire sample bulk. This is probably the first confirmed example of uniform microscopic coexistence of superconductivity and atomic ferromagnetism. The material is determined from thermopower measurements to be an underdoped cuprate with a projected T/sub c,max/=90-100 K, typical of a two-layer cuprate. The oxygen isotope effect exponent of /spl alpha//sub Tc/=1.7 is the largest observed in the high-T/sub c/ cuprates. These materials are expected to provide a rich source of new physics and applications.