Ruthenocuprates: Intrinsic magnetic multilayers
We report ac susceptibility measurements on polycrystalline samples of SrRuO_3 and three ruthenocuprates: superconducting RuSr_2GdCu_2O_8 (Ru-1212), superconducting RuSr_2Eu_(2-x)Ce_xCu_2O_y (Ru-1222, x=0.5) and nonsuperconducting, insulating RuSr_2Eu_(2-x)Ce_xCu_2O_z (Ru-1222, x=1.0). Ac susceptibility of both Ru-1222 compositions exhibit logarithmic time relaxation and `inverted' hysteresis loops. Ru-1212 samples exhibit none of these behaviors. We interpret the magnetic behavior of Ru-1222 in the framework of weakly coupled magnetic multilayers and argue that superconductivity coexists with qualitatively different magnetic behaviors.