Physical properties of the magneto-superconducting R_2-XCe_XRuSr₂Cu₂O_10-δ compounds

The superconducting (SC) R_2-XCe_XRuSr₂Cu₂O_10-δ (R= Eu and Gd) materials (T_C approx. 36 and 42 K for x=0.5) are also weak-ferromagnets with T_M approx. 124 and 180 K respectively. T_M ≥ T_C, a trend which is contrary to that obtained in most 'magnetic-SC' intermetallic systems. Various experimental techniques show that magnetism is due to the Ru sublattice, whereas SC is confined to the CuO₂ planes. The hole doping in these planes, with resulting metallic behavior and SC, and be achieved with appropriate variation of the R/Ce ratio. Among many interesting features, irreversibility phenomena at low magnetic fields and spin reorientation transition are observed. These probably originate from antisymmetric exchange coupling of the Dzyaloshinsky-Moria type. Below T_C, the decay of the remanent moment as a function of temperature and/or time is logarithmic. On the other hand, above T_C a non-logarithmic relaxation rate of the remanent moment is observed. Vortices are present in the samples in equilibrium without an external field, indicating the predicted 'spontaneous vortex phase'.