MAGNETIC-FLUX DYNAMICS IN INHOMOGENEOUS HIGH-TC SUPERCONDUCTORS IN WEAK FIELDS

Features of magnetic flux dynamics in samples of the high temperature superconductor YBaCu3Ox have been experimentally investigated in weak magnetic fields (10(3)-1 Oe) and at 77 K. The experiments were carried out in a simple arrangement including a field coil, a flat sample per pendicular to the field, and an RF-SQUID along a common axis. Samples of both high and low quality thin films, as well as polycrystalline ce ramics were investigated. Attention is focused on the inhomogeneous th in film, which show unusual ''flux jumps'' for small field changes. We find strong evidence for abrupt penetration of flux to the center of the sample. Such behavior, which would be expected within ''spin-glass '' like models, is also found in other inhomogeneous type-II supercond uctors.