Magnetic force microscopy of vortex pinning at grain boundaries in superconducting thin films

We succeeded to image with magnetic force microscopy individual vortices in thin Nb films as well as in thin YBa₂Cu₃O_7-δ (YBCO) films. Relying on the imaging of microfabricated gold loops, we are able to identify the exact location of the vortices within 10 nm with respect to topographic features. The center of most of the vortices is located in between the grains appearing at the film surface. For the Nb films with a larger coherence length, the pinning can be linked to the reduced film thickness in between the protruding grains. For the YBCO films with a very small coherence length, the pinning is likely to be dominated by line defects at the trenches in between the growth islands. On the other hand, Fourier analysis of the imaged vortex distribution shows that for thicker Nb films not all of the vortices are pinned in between the grains. Due to the vortex-vortex repulsion, the vortex lattice reveals short-range correlations which become more pronounced at higher fields.