Magnetic force microscopy with 25 nm resolution

We describe a refined technique for magnetic force microscopy (MFM) which makes possible magnetic imaging with 25 nm resolution. Previous MFM work has relied on a servo system which used the amplitude of the response of an electropolished iron wire cantilever tip to an applied, near-resonance excitation to maintain a constant force gradient between tip and sample. The present results have been obtained with a redesigned instrument which uses a fast lock-in detector to servo on the phase, as opposed to the amplitude of the tip response. With these changes and the use of sharp electrochemically etched Ni tips, it is possible to servo stably on the very weak force gradients encountered in MFM, within 20 nm of the sample surface, and thus obtain 25 nm resolution.