Surface superconductivity and matching effect in a niobium thin film

The flux pinning of a 120 nm thick polycrystalline niobium film has been investigated dynamically with the vibrating reed technique and, statically, with torque magnetometry. Weakly temperature dependent maxima were observed by torque magnetometry consistent with the matching fields for formation and splitting of vortex chains. From these measurements, we have determined the volume pinning force as a function of applied field which shows a non-monotonic behavior compatible with different vortex regimes. We found that the formation and splitting of vortex chains has negligible influence on the elastic coupling of the vortex lattice measured with the vibrating reed. Both techniques showed sharp maxima in the angular dependence of the upper critical field in agreement with the appearance of surface superconductivity. The elastic coupling between flux lines and film as well as the energy dissipation are finite at magnetic fields higher than the bulk critical field B-c2 suggesting the existence of a vortex structure in this field region. (C) 1998 Elsevier Science B.V. All rights reserved.