Magnetic properties of superconducting alloy films

Abstract

We have made torque magnetometer studies of superconducting films of up to 6 at.% indium in tin. The general structure of the magnetization curves is similar to that observed earlier for tin and indium films; in particular, the curves are highly irreversible. In the parallel field case the measurement is of the perpendicular moment Mperpendicular induced by a small perpendicular field Hperpendicular. We suggest that the appropriate way to describe the parallel field curves is by the Bean model with a critical current decreasing as the applied field increases. This seems to agree with the general shape of our experimental curves. Near to the critical field Hcparallel Deltour and Tinkham have proposed Jc is proportional to 1 -(Hparallel/Hcparallel)2 for films which have a second-order transition on to the normal state at Hcparallel. Not all of our data agree with this form for Jc, and we use a simple model of a pinning centre (a local thin spot) to suggest that Jc may decrease more rapidly near Hcparallel. The behaviour of the film critical fields is generally in agreement with theory except that the perpendicular critical fields are about 20% too high.