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

To test the influence of an rf magnetic field on pinning and depinning of single vortices in thin niobium films we prepared dc SQUIDs with a square loop of 1 /spl mu/m line width and a hole size of 20 /spl mu/m/spl times/20 /spl mu/m. On one side the niobium strip widens to an area of 5 /spl mu/m/spl times/5 /spl mu/m, which is large enough for the pinning of at least one vortex. The motion of the trapped vortices 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 level 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 sites, one can deduce the difference of the pinning energies and the activation energy for hopping between the pinning sites. By the use of a high frequency demagnetisation process the vortex can be forced to change the pinning site or to leave the superconductor, depending on the power and the frequency of the rf field.