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

Abstract

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}$^{\QTR{it}{ms}}$=30 K) and low magnetic field (\QTR{it}{H}$^{ext}$<30 Oe), corresponding to a bulk Meissner-phase. At intermediate temperatures, \QTR{it}{T}$^{\QTR{it}{ms}}$<\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.