Anisotropic Flux Penetration in Superconducting Nb Films With Frozen-in In-plane Magnetic Fields

The magnetic anisotropy of superconducting niobium films of different thickness was investigated using magneto-optical imaging. A wide range of anisotropic behaviors was generated by freezing in different in-plane external magnetic fields with magnitude up to 1.12 kOe. The critical fields for the parallel configuration of the different samples were calculated, taking into account the strong deviation of the superconducting properties at small thicknesses. Above their lower in-plane critical field, the films are in the mixed state containing coplanar vortices. In structurally isotropic Nb films, when applying a perpendicular magnetic field, the frozen-in vortices were seen to guide the perpendicular ones. It is shown that the anisotropy increases with both the film thickness and the magnitude of the in-plane field. The results demonstrate that the field-induced anisotropy is a highly versatile way to manipulate vortex matter in film superconductors.