  A number of aspects of magnetic force microscopy (MFM) specific to the
imaging of hard magnetic films have been studied. Firstly, we show that
topographic images made in tapping mode with probes characterized by the
moderate cantilever stiffness usual for MFM (1--4 N/m), contain artifacts due
to strong probe-sample interactions which lead to probe retraction. As a
result, stiffer cantilevers (e.g. 40 N/m) are better adapted to characterizing
such hard magnetic films. Secondly, imaging with probes coated by a hard
magnetic film leads to phase maps which show a twofold symmetry, with paired
dark/light contrast on opposite domain edges along the direction of the
cantilever. This is due to the tilt of the direction of tip magnetizaiton and
direction of oscillation, with respect to the sample normal. Thirdly, due to
the long-range nature of the stray field produced by hard magnetic films
containing micron-sized domains, MFM phase contrast reflects the stray field
itself, as opposed to that of its spatial derivatives, as is generally the case
in MFM.
