Theory of Flux Creep in Hard Superconductors

Previous investigations of the critical state of the hard superconductor have defined it in terms of constants alpha and B/sub O/; alpha (T) = J/sub cr/ (B/sub cr/ + B/sub 0/). If the current density J or the field B is increased beyond the critical values, a process called flux creep'' sets in, and flux leaks through the material and returns it to the critical state. alpha (T) is a structure-sensitive constant of the material. A theory is presented to show that this behavior of alpha can be explained by assuming that the mechanism of flux creep is thermally activated motion of bundles of flux lines, aided by the Lorentz force, over free energy barriers coming from the pinning effect of inhomogeneities, strains, dislocations, or other physical defects. The theory also explains B/sub 0/, and predicts time relaxation behavior similar to those of magnetic aftereffect and some forms of plastic creep. (H.D.R.)