Nanowire magnetic force microscopy

Local imaging of surface stray fields on the nanoscale is fundamental for understanding magnetism. With advancements in fabrication techniques a plethora two-dimensional magnetic materials are now readily available. By design they call for ultrasensitive imaging techniques, since their reduced volume leads to relatively weak magnetic stray fields.
One of the established magnetic imaging technique on the nanoscale is magnetic force microscopy (MFM). It employs micron-sized cantilevers as mechanical force transducers and offers the desired spatial resolution. However, it is lacking in terms of field sensitivity. Additionally, conventional MFM cantilever tips contain a considerable amount of hard magnetic material and hence can quickly disturb the subtle magnetization of weak magnetic samples.
This project focuses on improving the field sensitivity by employing nanowires (NWs) as magnetic force microscopy probes. Free standing NWs in the pendulum geometry naturally present themselves as extremely sensitive mechanical force sensors. Their high aspect ratios and low masses immediately lead to thermally limited force sensitivities of at least two orders of magnitude better than those of standard MFM cantilevers. In order to achieve magnetic imaging contrast we make use of focused electron beam induced deposition (FEBID) of cobalt, resulting either in a fully magnetic NW or a magetic tip on top of an existing structure.
We find that the magnetic image formation for a sufficiently long cylindrical Co tip can be approximated by a simple point-pole model, where only the magnetic surface charge at the free extremity of the tip needs to be considered. As a consequence the NWs' frequency shifts are proportional to the in-plane magnetic field gradients. In terms of field sensitivity the best characterized sensor achieves a value slightly better than 2 nT/√Hz at resonance.
These results present a significant improvement in field sensitivity and pave the way for ultrasensitive magnetic stray field and dissipation imaging at the nanoscale.