Overcritical states of a superconductor strip in a magnetic environment

A current-carrying superconducting strip partly penetrated by magnetic flux and surrounded by a bulk magnet of high permeability is considered. Two ty pes of samples are studied: those with critical current controlled by an ed ge barrier dominating over the pinning, and those with high pinning-mediate d critical current masking the edge barrier. It is shown for both cases tha t the current distribution in a central flux-free part of the strip is stro ngly affected by the actual shape of the magnetic surroundings. Explicit an alytical solutions for the sheet current and self-field distributions are o btained which show that, depending on the geometry, the effect may suppress the total loss-free transport current of the strip or enhance it by orders of magnitude. The effect depends strongly on the shape of the magnet and i ts distance to the superconductor but only weakly on the magnetic permeabil ity.