Depinning of single vortices in niobium thin film dc superconducting quantum interference devices by rf demagnetisation

To test the influence of an rf magnetic field on Dinning and depinning of s ingle vortices in thin niobium films we prepared de SQUIDs with a square lo op of 1 mu m line width and a hole size of 20 mu m X 20 mu m. On one side t he niobium strip widens to an area of 5 mu m x 5 mu m, which is large enoug h for the pinning of at least one vortex. The motion of the trapped vortice s from one pinning site to another generates flux changes in the SQUID loop and can thus be detected by the SQUID. We report on measurements of two le vel fluctuations caused by vortices hopping between different pinning sites . The investigations were made in a temperature range from 2.8 K to 4.5 K. By measuring the time, for which a vortex remains in one of the pinning sit es, one can deduce the difference of the pinning energies and the activatio n energy for hopping between the pinning sites. By the use of a high freque ncy demagnetisation process the vortex can be forced to change the pinning site or to leave the superconductor, depending on the power and the frequen cy of the rf field.