Scanning Hall probe microscope images of field penetration into niobium films

A high resolution scanning Hall probe microscope has been used to study the penetration of magnetic flux into thin strips of superconducting niobium as the applied field is slowly ramped. The strips, with widths w = 100 mu m, and thicknesses d approximate to 1 mu m, are thick enough such that vortices are truly three dimensional (d much greater than lambda). However, the small ratio d/w implies very strong demagnetization effects, and the relative smallness of d emphasizes the importance of the long-range force between Vortex ends over the short-range force between their bulk core currents. The microscope has 1-2 mu m spatial resolution and around 30 mG field sensitivity, allowing high-resolution imaging of flux features over its approx. 150 x 150 mu m(2) scan range. At low fields of a few tens of gauss, we observe Meissner screening of the external field. As the field is increased towards several kilogauss, flux begins to enter the sample in the form of small (approximate to 10 mu m wide) dendritic fingers. These fingers persist over all temperatures investigated, from 0.3 to 0.95 T-c, They appear to grow in such a way as to maximize their separation from neighbouring fingers. This suggests a growth mechanism of the flux front mediated by a competition between long-range repulsive interactions between mesoscopic flux-containing regions, and the strong pinning that maintains the stability of the flux front. (C) 2000 Elsevier Science B.V. All rights reserved.