Shaking of the critical state by a small transverse ac field can cause rapid relaxation in superconductors

In thin superconducting platelets or films in a perpendicular dc magnetic field, the application of a weak in-plane ac magnetic field perpendicular to the dc field causes the irreversible currents to relax rapidly. This 'vortex shaking' effect allows one to measure the reversible magnetic moment in spite of vortex pinning. We explain it first for a long strip with the in-plane ac field perpendicular or parallel to the strip axis (transverse or longitudinal shaking) and then for realistic rectangular platelets with any side ratio. In such rectangles the different relaxation rates of currents flowing in different directions lead to the appearance of a novel spatial structure with multiple current loops.