Magnetization relaxation of exchange biased (Pt/Co)5/Pt/IrMn multilayers withperpendicular anisotropy was investigated by time-resolved Kerr microscopy.Magnetization reversal occurs by nucleation and domain wall propagation forboth descending and ascending applied fields, but a much larger nucleationdensity is observed for the descending branch, where the field is appliedantiparallel to the exchange bias field direction. These results can beexplained by taking into account the presence of local inhomogeneities of theexchange bias field.
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