Coexistence of ferromagnetism and superconductivity in magnetic superconductor RuSr2YCu2O8 revealed by -NMR

63 Cu -NMR measurement has been performed to investigate the coexistence of superconductivity and ferromagnetism in a hybrid ruthenium (Ru)–copper (Cu) oxide superconductor RuSr2YCu2O8 (RuY1212). Broadening of the Cu-NMR signal below TM demonstrates that Cu nuclei in CuO2 planes feel a dipolar field from ordered Ru moments in RuO2 planes. On the other hand, the opening of the SC gap is confirmed from the observation of a decrease in the nuclear spin–lattice relaxation rate, 63 1/T 1 , below Tconset=45 K. These NMR results provide evidence that the superconductivity coexists with the (field induced) ferromagnetism at a microscopic scale. We also found that the T dependence and the magnitude of 1/T1T are quite similar to those in YBa2Cu3O7 (Y1237), indicative of the same spin-fluctuation character in both compounds. This suggests that the hole content in RuY1212 is comparable to that in Y1237.