Synthetic antiferromagnets (SAFs) are of interest in a wide variety of spintronic applications owing to their net zero magnetisation and inherent tunability arising from control over complex stacking sequences 1. They show very fast domain wall (DW) motion in response to spin torques 2.
Here we describe measurements of the current-driven DW motion in each of the separate magnetic sublattices of a SAF with structure Ta/Ru/Pt/CoB/Ru/Pt/CoFeB×5/Ru/Pt which displays both perpendicular magnetic anisotropy (PMA) and the Dzyaloshinskii-Moriya interaction (DMI). The thickness of the Ru layers is tuned to provide AF coupling between each pair of layers. The thicknesses of the alternating CoB and CoFeB layers are tuned to give net zero magnetisation (as measured by SQUID magnetometry) at a wide range of fields up to ~50 mT.
The magnetic films were grown on Si3N4 membranes and imaged with scanning transmission x-ray microscopy (STXM). They were patterned into 2 µm wide wires with Cu leads attached to each end. The distinct chemical composition of the two sublattices means that we could use the element specificity of STXM to observe the CoFeB separately by imaging at the Fe L3 edge, whilst contrast at the Co L3 edge is dominated by the other sublattice of CoB layers. Inverted domain structures observed at these two edges with XMCD-STXM are shown in Fig. 1, confirm the SAF ordering of the layers. Current pulses drive domain wall motion, as shown in Fig. 2, with average velocities up to ~ 40 m/s achieved at current densities as low as 3 × 1011 A/m2. These results demonstrate a low depinning current and fast motion compared to comparable FM multilayers we have previously studied, suggesting that domain walls in SAF heterostructures are superior to their ferromagnetic equivalents for low energy consumption racetrack technologies.
1 R. A. Duine et al., Nature Phys. 14, 217 (2018) 2 S.-H. Yang et al., Nature Nanotech. 10, 221 (2015)