Granular superconductivity in polycrystalline ruthenocuprateRuSr2(Gd1.5Ce0.5)Cu2O10−δ: magnetoresistive and magnetization studies

Abstract

Granular superconductivity effects in polycrystalline samples ofRuSr2(Gd1.5Ce0.5)Cu2O10−δ, as prepared (by a solid-state reaction method) and annealed (12 h at845 °C) in pure oxygen at 30 atm, are presented. The resistive transition to the superconductingstate of the as-prepared sample is found to be considerably affected by granularity.In particular, an evident kink in the temperature dependence of the resistanceR(T) is seen at thetemperature, Tc0≈34 K, at which grains become superconducting. The resistivetransition depends strongly on the applied current. The family ofR(T) curves taken for different transport currents is branched with a branching point atTcJ≈23.2 K. Below this temperature the intergrain Josephson coupling starts to develop. For low current,R decreases with decreasing temperature belowTcJ as expected for the transition to the superconducting state, whereasR(T) curves for higher current form a minimum atT≈17.3 K, showing a quasi-re-entrant behaviour. The influence of the granular structure of theas-prepared sample shows itself also in the temperature behaviour of the magnetization,M(T), in lowfield. Application of low magnetic field (below 400 Oe) leads to a broadening of the resistive transitionsbelow TcJ, similar to that caused by increasing the current. Both the currentand magnetic field depress the Josephson coupling between the grains,producing a dramatically large effect on the resistive transition. TheR(T) and M(T) dependences of the annealed sample show a fairly sharp superconducting transition far lessaffected by granularity. The results obtained imply that oxygen annealing improves theintergranular connection considerably, but it does not exert much influence on theintragrain superconductivity. No indication of intragrain granularity has been found in thesamples studied. The influence of ageing (due to deoxidation) of samples for differentconditions of storage is considered briefly as well.