Evidence for a bulk Meissner state in the ferromagnetic superconductor RuSr2GdCu2O8 from dc magnetization

From magnetization measurements we provide evidence that the ferromagnetic superconductor RuSr2GdCu2O8 with T-c approximate to 45 K and T-M approximate to 135 K; exhibits a sizeable diamagnetic signal at low temperature (T-ms = 30 K) and low magnetic field (H-ext<30 Oe), corresponding to a bulk Meissner phase. We argue that a recent negative report [C. W. Chu et at, cond-mat/9910056 (unpublished)] regarding the Meissner effect in Ru-1212 can be explained by impurity scattering or grain-size effects. At intermediate temperatures, T-ms (c), we observe unique thermal hysteresis effects which are characteristic of a spontaneous vortex phase.