Magnetic hysteresis in superconductors.

Magnetic hysteresis in rod specimens of type I superconductors and low K type II superconducting alloys has been studied. Magnetisation measurements with single crystal 2 wt% Pbln specimens, some of which were plastically strained at 4.2°K, showed that bulk flux pinning accounted for only a minor fraction of mixed state hysteresis. The major fraction was due to complete surface current shielding; this and the remaining contribution to hysteresis were both dependent on surface condition. Axial variations of magnetisation, dependent on field history and surface condition, showed that bulk flux changes were delayed by surface flux spot pinning. Theoretical considerations indicate a correlation between the surface shielding current hysteresis and flux spot pinning. Complex permeability transitions indicated a frequency effect in the response of 2 wt% Pbln to a small A. C. field. M waveforms and M - H minor hysteresis loops showed that this effect was evident only in that part of the hysteresis not associated with complete surface current shielding. The response to an incremental field change was analysed into a short-lived exponential, caused by bulk flux flow, and a longer lived transient, which, being dependent on surface condition, was tentatively associated with the viscous flow of flux spots. This mechanism was suggested as the origin of the frequency effects. The hysteresis in the magnetisation curves of unstrained specimens of Pb, Sn and In was almost eliminated by a sufficiently high plastic strain at 4.2°K. At lower strains, specimens under tension exhibited less hysteresis than unloaded specimens. These results are discussed in terms of the propagation of the superconducting phase into the deformed metal. Changes in the shape of the increasing field magnetisation curves of Hg specimens, strained at 4.2°K, indicated that the metal underwent a partial phase transformation. The product phase, labelled gamma Hg, is distinct from beta Hg by having a different critical field curve and a different temperature at which it transforms back to the alpha phase