Dendritic flux avalanches in superconducting Nb_3Sn films

摘要

The penetration of magnetic flux into a thin superconducting film of Nb_3Sn with critical temperature 17.5 K and critical current density 6 MA/cm~2 was visualized using magneto-optical imaging. Below 8 K an avalanche-like flux penetration in form of big and branching dendritic structures was observed in response to increasing perpendicular applied field. When a growing dendritic branch meets a linear defect in the film, several scenarios were observed: the branch can turn and propagate along the defect, continue propagation right through it, or "tunnel" along a flux-filled defect and continue growth from its other end. The avalanches manifest themselves in numerous small and random jumps found in the magnetization curve.