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

Recently it was shown that with thin superconductors placed in a transverse do magnetic field, the application of a small ac magnetic field perpendicular to the do field causes rapid relaxation of the irreversible magnetization. We show here that for a thin superconducting strip in a transverse do field the application of a weak ac field perpendicular to the do field generates a do 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.