Phenomenological description of flux pinning in non-uniform high-temperaturesuperconductors in magnetic fields lower than the self-field

Abstract

Critical currents measured on a commercial Bi-2223/Ag tape in applied magnetic fieldsranging from zero to 100 mT are analysed in order to demonstrate the validity of thephenomenological description proposed by the authors for flux pinning in polycrystallinehigh-temperature superconductors in low magnetic fields. The four-parameter model is ageneralization of the Kim formula describing the magnetic field dependence of the criticalcurrent density. When combined with the finite-element calculation, it is able to predict thecritical currents measured in different field orientations with an average accuracy betterthan 0.2%. The volume density of the flux pinning force is easily evaluated from theparameters of the model. Particular attention is paid to the problem of a possible samplenon-uniformity. In fact, the experimental data can be better explained assuming thehypothesis of better filament texturing in outer filaments. This also means that theaccurate determination of critical currents in low magnetic fields can be used toreveal a non-uniformity in the current-carrying capability of the superconductingmaterial.