CRITICAL-CURRENT RELAXATION IN CERAMIC SUPERCONDUCTORS - EFFECT OF THE SURFACE-BARRIER

The effect of the Bean-Livingston surface barrier on the thermoactivat ed relaxation of magnetization and critical current in superconducting ceramics is analyzed. The following picture of flux dynamics is discu ssed. At first, magnetic flux penetrates into intergranular regions, a nd then flux lines overcome the barrier and enter the granules owing t o thermally assisted flux motion. Such a thermal relaxation appreciabl y affects the magnetic flux distribution giving rise to time dependenc e of the intergranular magnetic field H-0. The variation of H-0 result s in the relaxation of the critical current j(c). The relaxation rates of H-0 and j(c) in increasing and decreasing magnetic field are calcu lated. These rates are shown to have clearly pronounced hysteretic beh avior due to asymmetry in flux entering and leaving the granules.