Strong in-plane magnetic field pulses of 2-4.4 ps duration are used to
study magnetization reversal in perpendicularly magnetized Co/Pt film
s. Ring domains, reminiscent of the field contour during exposure, are
observed later with Kerr microscopy. Their radii represent switching
fields which are in quantitative agreement with the coherent rotation
model. The observation of intrinsic transition broadening is attribute
d to the existence of static and dynamic fluctuations of the magnetic
anisotropy. [S0031-9007(98)07344-X].