Flux Pinning by Phase Boundaries in Type-II Superconductors

Measurements have been made, from magnetization curves, of the surface hysteresis in PbTl in low fields. The surface was plated with Tl and diffused for various periods so that the distance in which the specimen changed from normal to superconducting varied from a few atomic diameters to several times the penetration depth. It was found that plating had little effect, but that the hysteresis dropped markedly on diffusing over a distance $\lambda$. The results indicate that magnetic pinning to the surface is the predominant factor, rather than surface "sheath" currents.