Surface Dependence of the Critical Pinning Current Density in Type-II Superconducting Foils

Abstract

Measurement of flux-flow curves in cold-worked Pb-Bi type-II superconducting foils indicates that for samples otherwise prepared in the same way, ${J}_{P}$, the critical current density required to initiate flow, is linear in the surface-to-volume ratio of the samples. This and other surface effects suggest that, for these samples, pinning is almost entirely at the surface. Surface irregularities as well as mechanical defects act as sources of surface pinning. Changes in these pinning sources, achieved either through modifying the method of sample preparation or by annealing the samples, change the slope of the curve of ${J}_{P}$ versus surface-to-volume ratio. The results are discussed in terms of Pearl's theoretical model, in which the electromagnetic region of a fluxoid spreads at the sample surface.