RELAXATION OF HYSTERESIS LOOP IN A SINGLE-CRYSTAL OF YBA2CU4O8

Magnetic relaxation has been studied for Y1Ba2Cu4O8 in order to look f or changes in the relaxation rates that arise because different pinnin g mechanisms dominate in different magnetic field regimes. In the low field regime where the Bean-Livingston surface barrier dominates, the vortices exit on a field decreasing branch with a relaxation rate abou t ten times faster than they enter on a corresponding field increasing branch. In high magnetic fields where bulk pinning dominates, the rel axation rates for field increasing and field decreasing are essentiall y the same.