Investigating the Static and Dynamic Properties of Skyrmions in a Synthetic Antiferromagnet

Skyrmions and domain walls have been heralded as novel information carriers for a new generation of magnetic storage devices. Skyrmions in particular have many applications due to their 2D nature, and so have also been proposed as components in novel computing devices beyond the classical Von Neumann architecture where they could function as neurons in brain inspired computing models. Synthetic antiferromagnets (SAFs) are an ideal carrier for these domains and skyrmions. Because of the cancellation of stray field and their antiferromagnetic coupling, these magnetic textures have been predicted to move faster than their ferromagnetic (FM) counterparts, without tilt in the case of domain walls, and negating the skyrmion Hall effect (SkHE) in the case of skyrmions. However, due to this cancellation of the stray field and magnetisation, detection of these textures remains extremely challenging.
Here the growth of SAF multilayers with DMI is presented. In order to optimise these stacks for X-ray magnetic circular dichroism (XMCD) experiments, they are grown with alternating magnetic layers of CoB and CoFeB. Pt is used as an underlayer for both magnets, as it mediates a strong spin-orbit torque (SOT), as well as helping to stabilise an interfacial Dzyaloshinskii-Moriya interaction (DMI). Layers of Ru between each magnetic layer also contribute to this DMI, and are tuned to the correct thickness to mediate a Ruderman-Kittel-Kasuya-Yosida (RKKY) style interaction that enforces antiferromagnetic coupling between the layers.
The transitions of these samples through the major features of their hysteresis loop are studied using a variety of complementary microscopy techniques. This is important to understand as these systems are not observed to exhibit the spin flop interaction, instead transitioning through a defect-driven bubble nucleation process reminiscent of a first-order phase transition. This is a key understanding in interpreting results presenting skyrmions in synthetic antiferromagnets at fields near the transition from SAF ordering to ferromagnetic (FM).
The current induced motion of domain walls (DWs) in these systems has been studied, and shown to be considerably faster than the equivalent FM textures. By leveraging the chemical selectivity of XMCD, the behaviour of these DWs was studied for the first time in fully compensated SAFs. Furthermore due to the presence of Pt as a SOT-carrier with an interface to every magnetic layer, these DWs depin at a current density of $(3.5 \pm 0.4)\times10^{11}$~A/m$^2$, an order of magnitude less than comparable results.
Finally, micromagnetic simulations have been used to study the microwave driven resonances of skyrmions in these SAF multilayers. The number of resonances is shown to increase as a function of the number of magnetic layers, and interestingly, the frequencies of these resonances are lowered in comparison to the simple bilayer case. The applications of this to skyrmion sensing in SAFs is discussed with a focus on how the breathing modes will change on the introduction of structural and thermal disorder. This result is important as it paves the way to using benchtop ferromagnetic resonance measurements as a means to detect the presence of skyrmions in a SAF.