Influence of domain wall interactions on nanosecond switching in magnetic tunnel junctions

Abstract

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...

Magnetic imaging with spin-polarized low-energy electron microscopy

“…Magnetic imaging of layered systems can provide crucial information that may not be accessible otherwise. For example, magnetic stray fields generated by domain walls in the hard layer of a tunnel junction can significantly reduce the energy barrier to nucleate domains in the soft layer during magnetization reversal [124]. In micron-sized, ellipseshaped spin valve structures, the magnetic stray field of the hard layer is also known to induce 360…”

Influence of topography and Co domain walls on the magnetization reversal of the FeNi layer inFeNi∕Al2O3∕CoRomanens1, Vogel2, Kuch3 et al. 2006Phys. Rev. BSelf Cite8081View full textAdd to dashboardCite

“…In this section we will show that also inhomogeneities in the magnetization of the hard layer, like the presence of magnetic domain walls, can influence the magnetization reversal of the soft layer. In a previous paper 12 , we have shown that a domain wall in the Co layer of sample II locally decreased the barrier for domain nucleation in the FeNi layer. We have performed time-resolved XMCD-PEEM measurements also on the other samples, in order to confirm that the preferential nucleation induced by domain wall stray fields is a general property of this type of trilayers.…”

“…This effect can be understood by taking into account the domain wall stray field. In our previous paper, 12 we have discussed the effect of this stray field. In particular, micromagnetic simulations have shown that the Co domain wall stray field is sufficient to create a region where the FeNi magnetization is tilted away from the easy axis, creating a so-called quasi-wall.…”

“…For example, domain walls in the soft magnetic layer can create stray fields that are large enough to influence the magnetization of the hard magnetic layer in soft FM/NM/hard FM trilayers, as shown by Thomas et al 7 More recently, several authors have shown direct evidence of the effect of a domain wall in one layer on the static magnetic configuration of the other layer. 8,9,10,11 In a recent paper, 12 we have used the element selectivity of XMCD-PEEM to study independently the magnetization of both magnetic layers in FeNi/Al 2 O 3 /Co trilayers, showing that domain walls in the hard magnetic layer locally decrease the nucleation barrier for the switching of the soft magnetic layer. In this paper, we have extended this study in order to obtain information on the relative influence of topography and domain wall stray fields on the nanosecond magnetization reversal in spin-valves and magnetic tunnel junctions.…”

Magnetism in reduced dimensions

“…Notice however, that even in this weak coupling limit dipolar fields may be locally much more intense than HN when domain walls occur, like during magnetization reversal [120]. This may lead to progressive demagnetization of the hard layer of spin valves [121] or nucleation of reversed domains in the vicinity of domain walls [122]. In the strong coupling limit HN is larger than both coercive fields, resulting in rigidly coupled layers.…”