Domains were imaged by magnetic force microscopy (MFM) on materials wh
ere the domain size exceeds the sample thickness by three orders of ma
gnitude. Selected samples are a magnetooptical medium and ultrathin co
balt films, all with perpendicular magnetization. A strong domain cont
rast is observed in both cases. This fact is confronted to the usual t
heory of MFM image formation, in which the stray field from one body (
tip or sample) is sensed by the other, without altering of the magneti
zation distributions. It is shown that the domain contrast in such ext
reme conditions cannot be explained with that theory. On the contrary,
Abraham and McDonald's model, which considers the response of the sam
ple to the tip field, is quantitatively compared to experiment. It is
shown to provide a good qualitative description of the results, but no
t a quantitative one, because of oversimplification. (C) 1997 American
Institute of Physics.