Evidence for possible phase separations in RuSr 2 (Gd,Ce) 2 Cu 2 O 10−δ

An unusual thermal-magnetic hysteresis was observed between a minor magnetic transition around 120 K and the main one at 80 K in superconducting ${\mathrm{RuSr}}_{2}(R,\mathrm{Ce}{)}_{2}{\mathrm{Cu}}_{2}{\mathrm{O}}_{10\ensuremath{-}\ensuremath{\delta}}$ (Ru1222R) samples, where $R=\mathrm{Gd}$ or Eu, down to a submicron length scale. The observation suggests a possible phase separation and is consistent with the very small but universal demagnetizing factor observed, which is difficult to reconcile with the canted-spin structure previously proposed. In such a scenario, the unusual superconducting properties of the Ru-based cuprates can also be understood naturally.