Evidence for a transition from a bulk Meissner-state to a spontaneous vortex phase in RuSr${}_2$GdCu${}_2$O${}_8$ from DC magnetisation measurements

From magnetisation measurements we provide evidence that the ferromagnetic superconductor RuSr${}_2$GdCu${}_2$O${}_8$ with \QTR{it}{T}${}_c$=45 K and \QTR{it}{T}${}_M$=137 K exhibits a sizeable diamagnetic signal at low temperature (\QTR{it}{T}<\QTR{it}{T}${}^{\text{QTR}itms}$=30 K) and low magnetic field (\QTR{it}{H}${}^{ext}$<30 Oe), corresponding to a bulk Meissner-phase. At intermediate temperatures, \QTR{it}{T}${}^{\text{QTR}itms}$<\QTR{it}{T}<\QTR{it}{T}${}_c$, a spontaneous vortex phase forms which is characterized by unique thermal hysteresis effects. We argue that a recent negative report [C.W. Chu et al., cond-mat/9910056] regarding the Meissner-effect in Ru-1212 can be explained by impurity scattering or grain size effects.