Hysteretic Dependence of Critical Current on Applied Magnetic Field and Magnetic Irreversmujty Line Crossings in Polycry
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HYSTERETIC DEPENDENCE OF CRITICAL CURRENT ON APPLIED MAGNETIC FIELD AND MAGNETIC IRREVERSIBILITY LINE CROSSINGS IN POLYCRYSTALLINE YBa 2 Cu 3O7 THOMAS R. ASKEW, RICHARD B. FLIPPEN E. I. Du Pont de Nemours & Company, Central Research and Development, P.O. Box 80357, Wilmington, DE 19880-0357 ABSTRACT The transport critical current-density Jc has been measured in high quality polycrystalline YBa 2 Cu 3 0 7 as a function of temperature and for various sweeps of sub-Tesla applied magnetic fields. Large increases over the virgin response were observed in Jc at fields of a few hundred gauss and are attributed to trapped flux in the samples. The amount of trapped flux was measured and correlated to the change at Jc at various temperatures. The position of the magnetic irreversibility line was measured by field induced absorption shift in AC susceptibility. The various data are discussed in the context of the weak link conduction model. INTRODUCTION The percolative nature of current flow in the high temperature superconductors was recognized by the pioneers of the field (1,2). Many early experiments found that the Jc observed in bulk polycrystalline samples was typically three orders of magnitude less than the 0.1-1 MAmp/cm 2 seen in oriented thin film samples. In addition, the already low polycrystalline Jc was further reduced in weak magnetic fields (-100 gauss), losing about two more orders at 77K. This reduction, and the high degree of conduction anisotropy seen by Dinger, et. al. (3), led Ekin and co-workers to suggest that the very low Jc was due to Josephson weak link behavior (4). Subsequent measurements across individual grain boundaries (5,6) directly observed the effects of grain misalignment and showed Josephson junction behavior when weak magnetic fields were applied. Despite numerous studies of Jc in polycrystalline high Tc superconductors (7,8) the hysteretic behavior of Jc has been rarely mentioned (9-12). The authors are aware of only one major study (9) and that was confined to silver clad wires at 77K. This phenomenon is of interest for several reasons: First, most large scale applications of these materials require Jc's far higher than those currently achieved in polycrystalline materials. If the "intrinsic" Jc of single crystals and thin films could be achieved in bulk samples many of these applications would be attractive. Any process which boosts Jc, particularly Jc in a magnetic field, is therefore of interest. Second, it is now realized that the application of high magnetic fields cause macroscopic supercurrents to become lossy not by quenching processes related to Hc, but rather by activation of flux motion at fields much below Hc 2 (7). First suggested by Muller (2), this idea was recently confirmed by Malozemoff and co-workers (13) using measurements of AC susceptibility and other techniques on single crystals. The locus of points in the field-temperature plane where this transition occurs is called the irreversibility line (IL). Recent measurements of this type on polycrystalline material show
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