Granular superconductivity in polycrystalline ruthenocuprate RuSr₂(Gd_1.5Ce_0.5)Cu₂O_10−δ: magnetoresistive and magnetization studies

Abstract

Granular superconductivity effects in polycrystalline samples of RuSr 2 (Gd 1.5 Ce 0.5)Cu 2 O 10−δ , as prepared (by a solid-state reaction method) and annealed (12 h at 845 • 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, T c0 ≈ 34 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 T cJ ≈ 23.2 K. 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 ≈ 17.3 K, showing a quasi-reentrant 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 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

Hysteresis and stepwise structure in MR curves of granular superconducting ruthenocuprates RuSr2(Gd1.5Ce0.5)Cu2O10−δ

“…Polycrystalline structure of the ruthenocuprates determines the pronounced granular superconductivity effects in these compounds [8,9,10,11]. The results found are quite similar to those known for granular high-T c superconductors, and can be considered in a similar way.…”

“…The Gd doped sample RuSr 2 Gd 1.5 Ce 0.5 Cu 2 O 10−δ (Ru1222-Gd) has been studied. The study is to some degree a continuation of our previous studies [10,11] of granular superconductivity effects in the Ru1222-Gd. We will describe here some new results not mentioned in previous articles [10,11].…”

“…The study is to some degree a continuation of our previous studies [10,11] of granular superconductivity effects in the Ru1222-Gd. We will describe here some new results not mentioned in previous articles [10,11]. In particular, a stepwise structure in MR curves taken with increasing and decreasing field has been found and discussed.…”

“…Only a small drop in resistance can be seen (if some part of the grains becomes well coupled). In other cases, a barely noticeable change in R(T ) curvature can be noted near T = T c0 [11]. Intergrain Josephson coupling is strengthened with further decreasing temperature below T c0 , so that in a complicated network of the weak links between grains some discontinuous zero-resistance paths (or clusters) appear causing further resistance drop.…”

“…The intergrain contacts of any grain can be of different types [13]. Granular superconductors have a widened (in contrast to that in homogeneous superconductors) temperature range of resistive transition to the superconducting state, with a "shouldered" form of resistive curves R(T ), which is also a feature of the ruthenocuprates [8,9,10,11]. The shouldered form of the resistive transition reflects the inhomogeneous state of a granular superconductor.…”