VORTEX FLUX-CREEP AND ITS HYSTERETIC CHANGES WITH MAGNETIC-FIELD IN ATYPE-II SUPERCONDUCTOR

Measurements were made of the time dependence of the vortex flux densi ty (B) of a grain-oriented YBa2Cu3O7 sample at 4.2 K in various fixed fields (H) on several hysteresis loops, with H and B along the c axis. It was found that whenever the hysteretic changes of H are reversed i n sign, the vortex flux creep (dB/dlog t) decreases very rapidly to ze ro, where it lingers before changing in sign. At the same turning valu es of H, the slope of the hysteresis loops (dB/dH) also goes to zero. Both these properties are attributable to a reversal of vortex motion which occurs at the turning values of H and causes a reversal of frict ional pinning forces. The reversed pinning forces inhibit any changes in the vortex population in the sample and, hence, any changes in B.