Dendritic flux patterns in MgB2 films

Magneto-optical studies of a c -oriented MgB2 film with a critical current density of 10^7 A/cm^2 demonstrate a breakdown of the critical state at temperatures below 10 K. Instead of conventional uniform and gradual flux penetration in an applied magnetic field, we observe an abrupt invasion of complex dendritic structures. When the applied field subsequently decreases, similar dendritic structures of the return flux penetrate the film. The static and dynamic properties of the dendrites are discussed.