MAGNETIC FORCE MICROSCOPY OF MIXED SYSTEMS: DOES IT WORK?

Magnetic force microscopy (MFM) is a variant of atomic force microscopy (AFM), in which a probe with a magnetic coating is used, conferring sensitivity to the magnetic fields of the sample. In order to obtain useful information, it is necessary to separate the magnetic forces acting on the probe from short and long range non-magnetic forces. A variety of methods have been described, but in the most commonly employed configuration, the surface topography is measured by normal AFM methods, followed by lifting the probe from the surface, and scanning at a fixed height above the sample, in order to remove short-range nonmagnetic forces acting on the probe. The magnetic fields are detected by measuring the phase shift of the oscillating probe (this is the so-called lift mode). This technique is widely employed to study large (>50nm) magnetic domains in flat surfaces, and provides useful information under these circumstances. Recent work has established that this method may also be appropriate to characterise the magnetic properties of superparamagnetic nanoparticles [1], although work on using MFM to characterize nanoparticles has been scarce until now, presumably due to the difficulties in detecting very small magnetic domains with this technique [2]. Unfortunately, there are