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 nm–87 nm) and different Tc (7.9 K–9.1 K) we studied how the vortex configuration changes when changing the applied magnetic field under field-cooled conditions (1 mT–5 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. Introduction. – With the advent of high-temperature superconductivity, the interest in obtaining materials which are able to carry very high superconducting current densities in the presence of a magnetic field has revived. In particular, the direct interplay between the critical current density and the specific defect structures has attracted a lot of attention [1]. During the last few years there also emerged a strong interest to understand the flux pinning properties of artificial defects introduced by microfabrication (holes) [2] or by ion irradiation (tracks) [3]. The experiments indicate a dramatic increase of the pinning when there is a matching between the flux line lattice and the underlying array of pinning centers. The matching effects are less pronounced for thin Nb films which tend to contain a rather high density of intrinsic, randomly distributed defects. Nevertheless, when the Nb film is covering an array of magnetic dots, the flux pinning process is considerably affected [4].