Investigation of magnetothermal and critical current hysteresis in polycrystals of low and high T(c) type II superconductors.

摘要

The model of Clem and Hao and others is extended to account for the enhancement of the Meissner effect observed in single crystals of hysteretic type II superconductors upon thermal cycling below T_c in static applied magnetic fields. Predictions are made about the features of the final closed thermal hysteretis loop achievable by extensive cycling and their dependence on the temperature limits T₁ and T₂ T_c chosen for the cycles.; A large variety of observations, by several workers, of a narrow peak of enhanced Meissner effect near T_c in polycrystalline type II superconductors upon slow warming in static applied fields after fast field cooling, are qualitatively and quantitatively accounted for by a model where we introduced the scheme developed above for single crystals into a weak-linked intergranular network (matrix). This “two tier” framework is then extended to describe the enhancement of the Meissner effect observed by Hyun by thermal cycling of weak-linked Nb₃Sn below T_c in a static field.; A simple framework is presented which quantitatively develops the proposal of Evetts and Glowacki that the superposition of the applied field H _a and the return field, H_r of the magnetized grains, is the cause of the hysteretic behaviour of I_c in weak-linked high T _c superconductors and the occurrence of a peak in I_c versus H_a descending and reascending, after an excursion to various values, denoted Hcycle, or after field cooling in different H cool. Observations by several workers on the dependence, of the position of four categories of peaks of I_c, on Hcycle and Hcool are reproduced by this model and yield estimates of the “compression” factor C in the linear dipole approximation, H _r = C Mg. We also show that, ratios of the measured plateau values for the position of these peaks, lead to an estimate for C which is independent of H·_g, the penetration field into the grains, and of the model chosen to calculate the dependence of the magnetization of the grains, M_g, on H_a.; Instead of the artificial pseudo-Josephson - junction expression generally used by other workers in the analysis of I_c hysteresis phenomena, we develop a family of formulae based on the critical state concept applied to idealized planar geometry. Exploiting an especially simple case from this family of formulae we reproduce a panoply of experimental curves of I_c versus H_a displayed in the literature and exhibiting a variety, of features. Analysis of the extensive data of List et al reveals that our approach leads to results in accord with observations whereas the Josephson junction format does not. (Abstract shortened by UMI.)