Why are flux avalanches deflected by a metallic layer?

Sudden avalanches of magnetic flux bursting into a superconducting sample are deflected from their trajectories when encountering a conductive layer deposited on top of the superconductor. Remarkably, in some cases the flux is totally excluded from the area covered by the conductive layer. Even if experimental evidence of the phenomenon has been available for a few years, there is currently no theoretical model to describe it. Moreover, the question whether a single vortex would also undergo deflection when entering the region covered by a metallic layer is still unanswered. In this work we use the magneto-optical imaging (MOI) technique, based on the Faraday effect, to show that a conductive layer (Cu) can repel flux avalanches triggered in an underlying superconducting film (Nb). We present a simple classical model that accounts for the deflection of a single vortex and considers a magnetic monopole approaching a semi-infinite conductive plane. This model suggests that electromagnetic braking, arising from the eddy currents induced in the metal due to the moving vortex, is an important mechanism responsible for avalanche deflection. Furthermore, we have found that early theoretical descriptions for the vortex damping enhancement due to the metallic sheet need a correction at large vortex velocities, where a decrease of the damping coefficient is expected.