Critical supercurrents and the pinning of vortices in commercial Ng-60 at% Ti

Abstract

Heavily cold worked Nb-60 at% Ti wires have been studied. The wires had previously been heat treated and characterized by electron microscopy (Neal et al 1971). The critical supercurrents have been measured as a function of applied magnetic field at temperatures between 4.2 K and the transition temperature, and interpreted in terms of the pinning force per unit volume on the vortex lattice. Empirically the pinning force is given by F_p=f(b)B_c2ⁿ (2c2 and f(b) depends on the particular heat treatment but not on the temperature at which the critical current is measured. Present pinning theories are reviewed but, as none account satisfactorily for the results, a simple model based on pinning by the cell walls of the defect structure has been developed. It depends upon the Ginsburg-Landau parameter being enhanced in the cell walls and fits the data for the material given the optimum heat treatment very well. It is suggested that the other heat treatments produced cell walls with a different internal structure such that the basic pinning interaction and hence f(b) and n are different.