DYNAMICS OF INDIVIDUAL VORTICES AND FLUX BUNDLES IN NB FILMS

The dynamics of flux vortices in 0.7 mum thick Nb films have been inve stigated using a linear array of 2 x 2 mum2 Hall probes that can resol ve single vortices. The flux vortices move into and out of the centre of the film in bundles. The flux bundle radius has been measured direc tly by a coincidence technique, and was found to decrease from 3.4 mum to 1.6 mum as temperature increased from 4.5 K to 6.5 K. We have obse rved the onset of flux creep driven by an applied DC current, thus det ermining the pinning force at individual pinning sites and establishin g that pinning force f(p) is-proportional-to (1 - T/T(c)n with n = 0.5 2 +/- 0.05. The temperature dependence of the flux bundle radius can b e described by the collective pinning theory of Larkin and Ovchinnikov , taking into account the measured temperature dependence of the pinni ng force.