Skyrmion motion in synthetic antiferromagnets and ferrimagnets driven by asymmetric spin wave emission

Skyrmions have been proposed as new information carriers in racetrack memory devices. To realize such devices, a small size, high speed of propagation, and minimal skyrmion Hall angle are required. Synthetic antiferromagnets (SAFs) present the ideal materials systems to realize these aims. In this work, we use micromagnetic simulations to propose a new method for manipulating them using exclusively global magnetic fields. An out-of-plane microwave field induces oscillations in the skyrmion radius, which in turn emits spin waves. When a static in-plane field is added, this breaks the symmetry of the skyrmions and causes asymmetric spin wave emission. This in turn drives the motion of the skyrmions, with the fastest velocities observed at the frequency of the intrinsic out-of-phase breathing mode of the pair of skyrmions. This behavior is investigated over a range of experimentally realistic antiferromagnetic interlayer exchange coupling strengths, and the results are compared to previous works. Through this we demonstrate the true effect of varying the exchange coupling strength and gain greater insight into the mechanism of skyrmion motion. In an attempt to more accurately reproduce experimental SAF samples, we investigate the effect of changing the ratio between the layer magnetizations in the SAF, and demonstrate that maximum velocity can be achieved when there is 100% compensation. We also investigate the differences between excitation mechanisms, using both electric fields and spin transfer torques to excite skyrmion motion. These results will help to inform the design of future novel computing architectures based on the dynamics of skyrmions in synthetic antiferromagnets.