Field dynamics and thermal activation in exchange-biased Co/Pt multilayers

The field sweep rate and temperature dependence of a perpendicular exchange-biased Co/Pt multilayers are studied. The reversal dynamics of the exchange bias in the multilayers is explained by a competition between thermally activated spin reversal in the IrMn film and domain wall nucleation in the Co/Pt multilayer. These opposite dynamical effects for multilayers with IrMn film on top are found to depend on the Co layer thickness, magnetic field annealing conditions and the use of a thin Pt insertion layer at the Co/IrMn interface. On the other hand, for multilayers with a direct contact between the IrMn film and the outermost Co layer, domain wall nucleation is found to be the dominant reversal mechanism. At high field sweep rates, thermally activated spin reversal in the IrMn film is reduced and this enhances the perpendicular exchange bias for films in which magnetization reversal proceeds through domain wall propagation (e.g. [Pt/Co]n/Pt/IrMn). The domain walls that nucleate on defect sites at the Co/IrMn interface exert a torque on the antiferromagnetic spin structure which then promotes spin reversal in the IrMn film and therefore reduces the perpendicular exchange bias.