Pinning of Flux Vortices in Type II Superconductors

Abstract

The superconducting properties of Pb-Bi alloys are reported. Measurements were made on the epsilon phase (a reversible material with K ∼ 12) and the eutectic, which is this epsilon phase with pure Bi precipitates. It is shown that the irreversible properties of the eutectic are consistent with the pinning of vortices in a reversible matrix of epsilon phase. The critical state is shown to be the same in measurements of magnetization, transverse critical currents and zero field critical currents, and it is extended to cover a composite ferromagnetic material. Experimentally it is found that the variation of critical current with flux density, microstructure and temperature is given by the expression J _c = 33S _v M(rev.)/B^1/2 amps/sq.cm. (S ^v is the phase boundary area per unit volume and M (rev.) the magnetization of the matrix.) There is strong evidence that the irreversibility is due to pinning at the phase boundaries of precipitates and it is found that the surface currents observed in the epsilon phase at low fields can be explained by the same mechanism. A theoretical explanation of the results is given by considering the magnetic interaction between vortices and the phase boundary. The attraction of a vortex to its image is shown to provide a pinning force of the above form and the constant of proportionality agrees within an order of magnitude.