Magnetic-force microscopy of vortices in thin niobium films: Correlation between the vortex distribution and the thickness-dependent film morphology

We demonstrate the possibility to reliably image vortices in superconducting Nb films with a low-temperature magnetic-force microscope. Our force microscope enables to monitor the surface topography as well, allowing to correlate the location of the vortices with specific topographic features. For Nb films of different thickness ($32\, {\rm nm} \mbox{--} 87\, {\rm nm}$) and different $T_\ab{c}$ ($7.9\, {\rm K} \mbox{--} 9.1\, {\rm K}$) we studied how the vortex configuration changes when changing the applied magnetic field under field-cooled conditions ($1\, {\rm mT} \mbox{--} 5\, {\rm mT}$). We find that the vortex pinning preferentially occurs in between the grains appearing at the film surface. This is consistent with a distribution of pinning centers which is governed by the columnar growth of the Nb films. For thicker Nb films the vortex arrangement is no longer dominated by pinning alone, and there appear short-range correlations in the vortex lattice. The short-range correlations are enhanced when increasing the applied magnetic field.