Imaging of vortices in conventional superconductors by magnetic force microscopy

We have imaged vortices in the conventional superconductors NbSe2 (crystal) and Nb (thin film) with a low temperature magnetic force microscope (MFM). The MFM detection is based on commercially available piezoresistive cantilevers. A considerably improved sensitivity (0.2 pN/nm) at 4.3 K has been obtained by using a higher flexural mode of the cantilevers. The operation at higher (2nd or 3rd) mechanical resonances improves the signal-to-noise ratio by a factor of 5. The improved sensitivity allows us to reduce the heat dissipation down to 0.05 mW in the cantilevers without lowering their performance, which is highly desirable for MFM applications at liquid helium temperatures. The magnetic tip coating was optimized by relying on Co/Au multilayers grown by oblique incidence molecular beam epitaxy. A magnetic field of 0.5-3 mT was used to induce the vortices under field-cooled conditions. In thin Nb films, we observed an irregular vortex arrangement and the imaged vortices are attached to individual pinning centers. On the cleaved surface of NbSe2 crystals, we observed an evolution from a disordered towards an ordered state of the vortex lattice. The possibility to image the Abrikosov vortex lattice in NbSe2 can be understood in terms of collective pinning effects. (C) 2000 Elsevier Science B.V. All rights reserved.