Force gradient mapping of domain wall structures in magnetite (abstract)

We have used a magnetic force microscope (MFM) in a new imaging mode to study domain walls in magnetite. The oscillation amplitude and phase of a vibrating cantilever were recorded versus cantilever tip-sample separation at each point in an x-y raster scan of a conventional MFM image. Amplitude and phase changes of the MFM cantilever depend on the topographic, interferometric, damping, and magnetic force gradients effects the cantilever experiences. For small scan areas, the magnetic force gradient acting between the tip and sample could be separated from the other interactions. This allowed quantitative extraction of the magnetic force gradients into a three-dimensional dataset called a force gradient map (FGM). FGMs were made over a number of samples; the work reported here focuses on domain wall structures in single crystal magnetite. The results of previous conventional MFM images and FGM images will be compared. One result was that the apparent resolution of the MFM tip was strongly dependent on the oscillation amplitude of the cantilever tip, implying that this must be considered when modelling MFM response. The data also suggest that FGMs may provide more sensitive tests of micromagnetic models than conventional MFM imaging of magnetite. This is for two reasons: (i) as mentioned above, FGMs allow the magnetic signal to be separated from the topographic, damping and interferometric effects and (ii) because FGMs sample the interactions between the tip and sample at a variety of well defined separations, it is possible to identify critical micromagnetic structural length scales.