Physical properties of the magneto-superconducting R2-XCeXRuSr2Cu2O10-δ compounds

The superconducting (SC) R2-xCexRuSr2Cu2O10-δ (R = Eu and Gd) materials (TC∼ 36 and 42 K for x = 0.5) are also weak-ferromagnets with TM∼ 124 and 180 K respectively. TM>>TC, 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, can 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 TC, the decay of the remanent moment as a function of temperature and/or time is logarithmic. On the other hand, above TC 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”.