Size of flux jumps in superconducting films

Magneto-optical imaging is used to visualize vortex avalanches in MgB₂ films at 40.3em0exK. Avalanches ranging from 50 to 50 000 vortices were detected. The size distribution function has a clear peak whose position moves towards larger sizes as the applied field increases. This field dependence as well as variation of flux density profile during an avalanche is well described by a proposed model assuming a thermal origin of the avalanches. The model is based on the adiabatic approach and takes into account nonlocal electrodynamics in thin superconductors. The threshold field for thermal avalanches is predicted to be much smaller than that for thick superconductors, in agreement with the experiment.