Understanding electrostatic and magnetic forces in magnetic force microscopy: towards single superparamagnetic nanoparticle resolution

The detection of superparamagnetic nanoparticles by magnetic force microscopy (MFM) at the single particle level faces difficulties such as superposition of nonmagnetic signals caused by electrostatic interactions as well as reaching the resolution limits due to small magnetic interactions. In MFM the magnetic force is measured at a certain distance to the substrate following the topography measured in a first scan to avoid an influence of short range forces (lift mode). In this work we showed that performing MFM on superparamagnetic nanoparticles the increase of the tip-substrate distance above the nanoparticle in lift mode scans leads to a reduction of the electrostatic forces resulting in a positive phase shift in contrast to the negative phase shift of the attractive magnetic force. Identifying the electrostatic force in MFM on nanoparticles as a capacitive coupling effect between tip and substrate the origin of often seen topography mirroring in phase images of nanoparticles in general is theoretically explained and experimentally proved. Minimization of the capacitive coupling by adjusting the work function difference between tip and substrate as well as using an optimized tip allows the magnetic visualization of single 10 nm superparamagnetic iron oxide nanoparticles (SPIONs) at ambient conditions with and without an external magnetic field. (paper)