Asymmetry of the Pinning Force in Thin Nb Films in Parallel Magnetic Field

The pinning force, Fp, is studied in Nb films of different thickness in parallel magnetic field H. The asymmetry in the magnetic field dependence of Fp has been observed for two opposite directions of the transport current. The effect is less pronounced for thin and thick films where, respectively, single vortex pinning and pinning of the internal vortices, is relevant. At intermediate thickness, where the pinning mechanism is mostly caused by surface effects, an asymmetry in the Fp(H) dependence is clearly visible. The different surface barriers that vortices should overcome to enter the sample from opposite sides of the film explain the effect, as confirmed by numerical calculations. These have been obtained by solving the Ginzburg–Landau equations with asymmetric boundary conditions which take into account the different superconducting properties of the film–substrate and film–vacuum interface. Such difference can also explain the reduction of the critical current usually observed in thin films as a function of their thickness.