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

Abstract

In thin superconducting platelets or films in a perpendicular dc magnetic field, theapplication of a weak in-plane ac magnetic field perpendicular to the dc field causes theirreversible currents to relax rapidly. This ‘vortex shaking’ effect allows one to measure thereversible magnetic moment in spite of vortex pinning. We explain it first for a long stripwith the in-plane ac field perpendicular or parallel to the strip axis (transverse orlongitudinal shaking) and then for realistic rectangular platelets with any side ratio. Insuch rectangles the different relaxation rates of currents flowing in different directionslead to the appearance of a novel spatial structure with multiple current loops.