Magneto-optical image of flux density avalanches in MgB2.
The penetration of magnetic flux into superconductors such as borides or borocarbides can happen in form of chaotic flux avalanches which occur due to local heating by dissipative flux line movement. These avalanches exhibit large velocities much larger than the phononic heat transport allows. Covering these superconducting films with metal layers it is possible to suppress the formation of these avalanches. It is unclear, why there are superconductor exhibiting avalanche behaviour and others (like YBCO) where no avalanches can be seen. Samples with a nonhomogeneous microstructure can shed light on possible processes wich are responsible for avalanche formation.