Granular superconductivity in polycrystalline ruthenocuprate RuSr ₂(Gd_1.5Ce_0.5)Cu₂O _10-δ: Magnetoresistive and magnetization studies

Granular superconductivity effects in polycrystalline samples of RuSr ₂(Gd_1.5Ce_0.5)Cu₂O _10-δ, as prepared (by a solid-state reaction method) and annealed (12h at 845 °C) in pure oxygen at 30atm, 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, T_c0≈34K, 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 T_cJ≈23.2K. Below this temperature the intergrain Josephson coupling starts to develop. For low current, R decreases with decreasing temperature below T_cJ as expected for the transition to the superconducting state, whereas R(T) curves for higher current form a minimum at T≈17.3K, 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 400Oe) leads to a broadening of the resistive transitions below T_cJ, 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.