Trapped flux in impure superconductive tin

Abstract

We have measured the percentage of trapped flux in cylindrical tin samples containing antimony, bismuth, or indium, with particular regard to the effect of annealing. The flux was trapped by the application and removal of a large transverse magnetic field. Our results do not confirm Pippard's conclusion that a marked change in flux trapping behavior occurs at a critical concentration of impurity. Instead we find that all samples, whether of high- or of low-impurity concentration, follow the same behavior, namely, that the percentage of trapped flux rises steeply near the transition temperature, and that this rise decreases monotonically with annealing time. In all cases in which this rise is discernible its temperature dependence is linear with the function (1-t4)-12 up to t>0.98, where t=TTc. We believe that the binary specimens used by us as well as those used by Pippard have a substructure of filaments of different concentration, which trap flux when the sample is insufficiently annealed. In support of this view we cite extensive metallurgical evidence for the existence of such a substructure, and a crude measurement on two of our annealed specimens which showed that the magnetic field needed to restore resistance was much higher than the threshold field of the bulk material, and that this transition was quite broad. © 1956 The American Physical Society.