Low temperature magnetic force microscopy with enhanced sensitivity based on piezoresistive detection

We describe the design and performance of a low temperature magnetic force microscope (MFM) based on commercially available piezoresistive cantilevers. The sensitivity has been increased by exciting the cantilever at a higher (second or third) flexural mode. The operation at higher mechanical resonances allows to improve the signal-to-noise ratio by a factor of 3. Our MFM is particularly advantageous for studying magnetic vortices on the surface of superconductors. The magnetic tip coating was optimized by relying on Co/Au multilayers grown by molecular beam epitaxy. This allows one to keep the interaction with the vortices small, and it becomes possible to observe a stable vortex lattice on the surface of a cleaved NbSe2 crystal. From our measurements of the disordered vortex state in thin Nb films we infer that the magnetic stray field induced by the tip is in the range 0.3–0.5 mT.