Role of RKKY torque on domain wall motion in synthetic antiferromagnetic nanowires with opposite spin Hall angles

S Krishnia,W L Gan,F N Kholid,I Purnama,T S Herng,J Ding,P Sethi,M Ramu,W S Lew

doi:10.1038/s41598-017-11733-9

S Krishnia, W L Gan + Show 7 more

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https://doi.org/10.1038/s41598-017-11733-9

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Abstract

We experimentally show the effect of enhanced spin-orbit and RKKY induced torques on the current-induced motion of a pair of domain walls (DWs), which are coupled antiferromagnetically in synthetic antiferromagnetic (SAF) nanowires. The torque from the spin Hall effect (SHE) rotates the Néel DWs pair into the transverse direction, which is due to the fact that heavy metals of opposite spin Hall angles are deposited at the top and the bottom ferromagnetic interfaces. The rotation of both DWs in non-collinear fashion largely perturbs the antiferromagnetic coupling, which in turn stimulates an enhanced interlayer RKKY exchange torque that improved the DW velocity. The interplay between the SHE-induced torque and the RKKY exchange torque is validated via micromagnetic simulations. In addition, the DW velocity can be further improved by increasing the RKKY exchange strength.

Highlights

Recent observations of fast current-driven domain wall (DW) motion under the influence of spin Hall effect (SHE) in ferromagnetic (FM) materials with perpendicular magnetic anisotropy (PMA) have led to the development of novel spintronic devices
The multiple magnetization reversal processes in the hysteresis can be attributed to the antiferromagnetic coupling between the bottom (M1) and the top (M2) magnetic Co/Ni/Co trilayers that are separated by an ultrathin Ru spacer layer
Observation of DW tilting in addition to high DW resistance is attributed to the presence of Dzyaloshinskii-Moriya Interaction (DMI) from the Pt underlayer and the Ta capping layers

Summary

Introduction

Recent observations of fast current-driven domain wall (DW) motion under the influence of spin Hall effect (SHE) in ferromagnetic (FM) materials with perpendicular magnetic anisotropy (PMA) have led to the development of novel spintronic devices. The current-induced DW motion in the SAF nanowires is explained by an additional torque which arises due to RKKY antiferromagnetic coupling[8]. We study the enhanced current-induced DW motion in a perpendicularly-magnetized SAF nanowire due to the high SHE and RKKY-induced torques. The DWs in the SAF nanowire become non-collinear due to the SHE-induced torques that act on the DWs in same direction at both the top and the bottom interfaces. This in turn stimulates an enhanced RKKY interlayer exchange torque which further improves the DW velocity as compared to the previous study[8]. Our calculations and micromagnetic simulations show that the magnitude of the RKKY exchange torque increases with respect to the perturbation in the antiferromagnetic coupling and it is maximum when both the DWs are perpendicular to each other

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