A MODEL FOR THE HYSTERETIC CRITICAL CURRENT-DENSITY IN POLYCRYSTALLINE HIGH-TEMPERATURE SUPERCONDUCTORS

The transport critical current density, J(c), of polycrystalline high-temperature superconductors has been found to be strongly hysteretic in an applied magnetic field. In order to explain the experimental data, a simple theoretical model is employed which is based on the demagnetization effect of grains in a granular superconductor. The demagnetization causes the magnetic field in the grain boundary Josephson junctions to become dependent on the grain magnetization which is strongly hysteretic resulting in an irreversible behaviour of J(c). The model explains quantitatively the observed shift of the J(c) peak to positive fields in a decreasing applied field for both zero field cooling and field cooling as well as the shift saturation for large maximal applied fields. The model further relates the observed broadening and height reduction of the J(c) peak to variations of the grain demagnetizing factor and the grain size in the sample.