Superconducting and magnetic behaviour of niobium doped RuSr2Gd1.5Ce0.5Cu2O10-δ

Polycrystalline samples of Ru1-xNbxSr2Gd1.5Ce0.5Cu2O10-delta, 0 <= x <= 0.5, have been synthesized and structurally characterized by x- ray diffraction ( XRD). Resistivity, magnetization and AC susceptibility measurements have been done and analysed considering a phase separation scenario. A strong suppression of the cluster glass ( CG) transition associated with niobium doping was identified. In fact, the CG phase was not present in samples for x >= 0.2, leading to changes in the magnetic hysteresis loops measured at low temperatures. These hysteresis loops can be explained as a result of the contribution of two distinct magnetic phases: the canted AFM phase and embedded Ru4+- rich clusters which order as a CG in low temperatures. Interestingly, the significant changes in the magnetic response of the material do affect the superconducting transition temperature Tc. It was found that both Tc and the superconducting fraction are reduced in samples which present the spin glass phase. Therefore, our results point to some coupling between magnetism and superconductivity in this ruthenocuprate family, the presence of the magnetic moment being deleterious for the superconductivity.