  Magnetic relaxation measurements were carried out by magneto-optical Kerr
effect on exchange biased (Pt/Co)5/Pt/FeMn multilayers with perpendicular
anisotropy. In these films the coercivity and the exchange bias field vary with
Pt spacer thickness, and have a maximum for 0.2 nm. Hysteresis loops do not
reveal important differences between the reversal for ascending and descending
fields. Relaxation measurements were fitted using Fatuzzo's model, which
assumes that reversal occurs by domain nucleation and domain wall propagation.
For 2 nm thick Pt spacer (no exchange bias) the reversal is dominated by domain
wall propagation starting from a few nucleation centers. For 0.2 nm Pt spacer
(maximum exchange bias) the reversal is strongly dominated by nucleation, and
no differences between the behaviour of the ascending and descending branches
can be observed. For 0.4 nm Pt spacer (weaker exchange bias) the nucleation
density becomes less important, and the measurements reveal a much stronger
density of nucleation centers in the descending branch.
