Relation of critical current irreversibility to trapped flux and microstructure in polycrystalline YBa2Cu3O7

Abstract

A study has been made of the critical current in silver clad YBa2Cu3O7 wires. Detailed resistive transitions, in zero magnetic field and with a magnetic field applied, indicate that below 10−3 V m−1 these materials display clear power law behaviour of the form E = kln with n values up to about 16. This characteristic is similar to that seen in conventional superconducting composite wires and has been related to the spatial distribution of inhomogeneity. For higher electric fields, E, there is a deviation from simple power law behaviour as current is progressively transferred to the silver cladding. It is shown that the variation of voltage with magnetic field at constant current can be related directly to lc(B), the variation of critical current with applied field. The dependence of lc(B) on the sintering conditions and amount of SiO2 impurity addition is presented and it is shown that lcversusB can be related to the Fraunhofer diffraction structure for a distribution of Josephson junctions in an applied field. It is further observed that lc(B) for all samples shows a striking irreversibility with cycles in the applied field. The Fraunhofer related lc(B) curve is displaced in B and the maximum lc reduced. The magnitude of the shift depends on the maximum value of B in the cycle and varies with sintering temperature and impurity content. An interpretation of the complex dependence of critical current on applied field and sample microstructure is given in terms of the hysteretic properties of a Josephson junction network mediated by trapped flux which is retained by strong pinning regions.