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

We have obtained microscopic evidence of the influence of domain wall stray fields on the nanosecond magnetization switching in magnetic trilayer systems. The nucleation barrier initiating the magnetic switching of the soft magnetic Fe 20 Ni 80 layer in magnetic tunnel junctionlike FeNi/ Al 2 O 3 / Co trilayers is considerably lowered by stray fields generated by domain walls present in the hard magnetic Co layer. This internal bias field can significantly increase the local switching speed of the soft layer. The effect is made visible using nanosecond time-and layer-resolved magnetic domain imaging and confirmed by micromagnetic simulations. DOI: 10.1103/PhysRevB.72.220402 PACS number͑s͒: 75.60.Jk, 75.60.Ch, 75.70.Ϫi, 85.70.Kh The active part of devices such as spin valves and magnetic tunnel junctions, used in magnetic random access memories ͑MRAM͒, consists of an ultrathin soft ferromagnetic ͑FM͒ layer and a harder ferromagnetic layer separated by a nonmagnetic ͑NM͒ spacer layer. These devices rely on the fast switching of the magnetization of the soft layer for reading or writing separate bits of information. Micromagnetic interactions have a strong influence on this switching. Demagnetizing effects and stray fields at the edges of nanosized magnetic structures can influence the magnetic configuration and the magnetization reversal of the soft magnetic layer, but interface roughness can also play a role and induce a magnetostatic coupling with the underlying hard magnetic layer. Much larger, but more localized magnetostatic effects exist when a domain wall is present in the hard magnetic layer.1,2 Direct evidence of the influence of domain wall stray fields in one layer on the static domain configuration of another layer has been obtained by Kuch et al. 3 on Co/ Cu/ Ni trilayers using x-ray photoelectron emission microscopy ͑X-PEEM͒. Schäfer et al. 4,5 have used Kerr microscopy to show the effect of stray fields of Bloch domain walls in an Fe whisker on the magnetization of a thin Fe film through a MgO spacer. Similar effects were recently also observed in systems with perpendicular magnetization.6 Thomas et al. 7 have observed that repeated motion of domain walls in the soft magnetic layer of a FM/ NM/ FM trilayer can demagnetize the hard magnetic layer, even if the field used for the reversal is much smaller than the coercive field of the hard layer. In thin films with in-plane uniaxial anisotropy the static coercivity is usually determined by the field needed for domain nucleation. In FM/ NM/ FM trilayers, the stray field of a domain wall in the hard magnetic layer can locally decrease this quasistatic nucleation field in the soft magnetic layer.1 In this paper we show a direct, real-time observation of this effect in Fe 20 Ni 80 /Al 2 O 3 / Co trilayers. In order to do so, we took advantage of the element selectivity of X-PEEM combined with x-ray magnetic circular dichroism ͑XMCD-PEEM͒. Our micromagnetic simulations show that the stray field of domain walls in the Co layer locally tilts the magneti...

International audienceSoft x-ray resonant magnetic scattering has been used to investigate the element-selective microscopic magnetization reversal behavior of room temperature perpendicular exchange coupled ferromagnetic/antiferromagnetic (F/AF) systems and to study the role of the interfacial coupling strength on it. Different nucleation processes and domain size distributions along the decreasing and increasing branches of the reversal have been found. The size of the magnetic domains during reversal depends on both the F anisotropy and F/AF coupling strength, decreasing when one of them increases. Evidence of the exchange bias (coercivity enhancement) being induced by pinned (unpinned) uncompensated AF interfacial spins is also shown