Reversible magnetic behavior of superconductors forced by a small transverse ac magnetic field

Abstract

Recently it was shown that with thin superconductors placed in a transverse dc magnetic field, the application of a small ac magnetic field perpendicular to the dc field causes rapid relaxation of the irreversible magnetization. We show here that for a thin superconducting strip in a transverse dc field the application of a weak ac field perpendicular to the dc field generates a dc voltage in the strip. This voltage leads to the decay of the critical currents circulating in the strip, and eventually the equilibrium state of the superconductor is established. This relaxation is not due to thermally activated flux creep but to the “walking” motion of vortices in the two-dimensional critical state of the strip with in-plane ac field. Our theory explains the “shaking effect” that was used for detecting phase transitions of the vortex lattice in superconductors with pinning. Some recent experiments are discussed.