Hysteretic dependence of critical current on applied magnetic field and magnetic irreversibility line crossings in polycrystalline YBa2Cu3

The transport critical density Jc has been measured in high quality polycrystalline YBa2Cu3O7 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