Granular superconductivity in polycrystalline ruthenocuprate RuSr2(Gd1.5Ce0.5)Cu2O10−δ: magnetoresistive and magnetization studies

Granular superconductivity effects in polycrystalline samples of RuSr2(Gd1.5Ce0.5)Cu2O10-delta, as prepared (by a solid-state reaction method) and annealed (12 h at 845 deg C) in pure oxygen at 30 atm, are presented. The resistive transition to the superconducting state of the as-prepared sample is found to be considerably affected by granularity. In particular, an evident kink in the temperature dependence of the resistance R(T) is seen at the temperature, Tc034 K, at which grains become superconducting. The resistive transition depends strongly on the applied current. The family of R(T) curves taken for different transport currents is branched with a branching point at TcJ23.2 K. Below this temperature the intergrain Josephson coupling starts to develop. For low current, R decreases with decreasing temperature below TcJ as expected for the transition to the superconducting state, whereas R(T) curves for higher current form a minimum at T17.3 K, showing a quasi-re-entrant behaviour. The influence of the granular structure of the as-prepared sample shows itself also in the temperature behaviour of the magnetization, M(T), in low field. Application of low magnetic field (below 400 Oe) leads to a broadening of the resistive transitions below TcJ, similar to that caused by increasing the current. Both the current and magnetic field depress the Josephson coupling between the grains, producing a dramatically large effect on the resistive transition. The R(T) and M(T) dependences of the annealed sample show a fairly sharp superconducting transition far less affected by granularity. The results obtained imply that oxygen annealing improves the intergranular connection considerably, but it does not exert much influence on the intragrain superconductivity. No indication of intragrain granularity has been found in the samples studied. The influence of ageing (due to deoxidation) of samples for different conditions of storage is considered briefly as well.