A high resolution, high sensitivity magnetic force microscope (MFM) with the ability to image in an in situ magnetic field will be described. This MFM has been used to investigate the micromagnetics of nanolithographically produced magnetic particles. It will be shown that the particles' switching field can be determined without being perturbed by the stray fields from the sensing tip. This allows the study of the evolution of the particles' magnetic states as a function of applied field and the direct observation of cooperative switching. The MFM has also been used, in conjunction with an external biasing field, as a localized source of flux for testing the stability of magnetic states and setting model magnetic configurations. MFM images of the particles are compared with numerical simulations. These comparisons have also provided insight into the magnetic behavior of the MFM sensing tips.
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