  The penetration of magnetic flux into a thin superconducting film of Nb3Sn
with critical temperature 17.8K and critical current density 6MA/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.
