FLUX-PENETRATION IN BI(2212) SINGLE-CRYSTALS AT LOW MAGNETIC-FIELDS

We report high-resolution measurements of the magnetization for a seri es of Bi(2212) single crystals during continuous heating up in a low m agnetic field after zero-field-cooling. We find a thermally activated behavior for the first flux penetration with a thermal activation ener gy decreasing logarithmically with increasing held below an internal f ield of about 150 Oe and being nearly field independent at higher fiel ds. This behavior suggests that thermal activation of pancake vortices across Bean-Livingston barriers at the surface determines the first f lux penetration at low fields, crossing over to a flux-flow process de termining the flux penetration at higher fields. In the flux-how regim e we can resolve large and irregular jumps in the magnetization, indic ative of a self-organized-critical (SOC) state.