Studies of the Intermediate State in Thin Superconducting Films

Studies of the intermediate state of tin and lead films, 2000 Å thick, in a transverse magnetic field have been made using a Hall-effect probe with a resolution of ¼ mm. The magnetization patterns showed structure comparable with the resolution at fields much smaller than the bulk critical field; it is likely that this was not the true domain structure but rather the coarse modulation of a much finer domain spacing due to lack of uniformity in the film. Except when flux trapping occurred, there was no sign of any structure much coarser than the resolution. The magnetization patterns were reproducible under similar conditions with identical magnetic history. As far as their magnetic properties were concerned, the films seemed to be completely normal at fields well below that at which resistance appeared, indicating the persistence of continuous superconducting threads. Hysteresis effects associated with flux trappings were observed, the pattern of the trapped flux being disturbed by the passage of a current. It was also found that a current could generate trapped flux in a virgin film. Measurements of the magnetic field configuration due to a current in a lead strip 2000 Å thick and 1.5 mm wide showed the current to be concentrated at the edges.