Thickness dependence of pinning mechanisms in granular Nb thin films

We have investigated the influence of thickness on the pinning properties of granular sputtered Nb films in a parallel magnetic field. The presence of a crossover in the pinning mechanisms at some critical thickness d(c) was established. Below d(c) the pinning is due to individual vortices. Above dc the commensurability between the vortex period and the film thickness occurs, causing a large increase in the critical current density due to surface pinning. It is shown that, in the vortex-free limit, when the critical current is determined by the depairing current, the effect of the commensurability is not present.