Patterns of Magnetic Flux Penetration in Superconducting Films

Intermediate-state magnetic flux patterns have been observed in thin superconducting films of tin, lead, and indium in perpendicular applied fields of 0.1-100 Oe. The spatial variation in flux density was measured very close to the film with a high-resolution Hall probe which could be scanned in a plane parallel to the surface of the film. The patterns appear to contain both laminar and round normal regions, with the latter predominant. A model for the intermediate state has been developed in which the normal regions are macroscopic flux spots. Experimental results for the number density of these flux spots in 1.1-6.3-mu-thick indium samples are in good agreement with the temperature, field, and thickness dependences of our model; indium samples thinner than 1 mu appear to enter the mixed state. In tin and lead, however, the flux patterns are strongly influenced by flux pinning. Appearance of the flux pattern at the critical field or temperature has been observed and correlated with distinctive features in the resistive transition. The extent of flux mu penetration into the sample was determined from measurement of the average field and compared for various field-temperature paths to the intermediate state. With the applied field reduced to zero, the magnetic field outside a current-carrying superconducting strip has been measured and indicates that the current distribution within the strip is consistent with the form predicted by Bowers.