Dynamics of synthetic antiferromagnetic skyrmion from current-induced deterministic motion to thermally-activated diffusive motion

Abstract

Magnetic skyrmions, topologically stabilized quasi-particles, have attracted much attention not only from fundamental physics but also from device applications1. Ferromagnetic skyrmions can be easily stabilized at room temperature by an interfacial Dzyaloshinskii-Moriya interaction (DMI) and can be efficiently controlled electrically through spin-orbit torques (SOT)2. However, problems arise from a large gyrotropic force due to the skyrmion Hall effect (SkHE)3, which is a diagonal motion along with current flow direction, that is easily able to surpass repulsive force from device edge. Thus, ferrimagnetic/antiferromagnetic skyrmions, which consist of two skyrmions antiferromagnetically coupled at each sublattice4,5, have received much attention due to a SkHE-free motion. Formation of ferrimagnetic6/synthetic antiferromagnetic (SyAFM) skyrmions7 has lately been demonstrated. Also, the SkHE-free motion of a half skyrmion is observed in a ferrimagnetic system at a specific temperature where angular momentum is compensated8. Earlier theoretical work has also predicted intriguing thermal characteristics of AF skyrmion including topology-dependent diffusive dynamics5,9. Nevertheless, the thermal effect on AF skyrmion is hitherto experimentally unexplored.Our work comprises two parts to comprehensively cover the full dynamics range. The first one is about the current-induced deterministic motion10, where we show the current-induced motion of skyrmions with suppressed SkHE even at room temperature in a SyAFM system. In the second part, we demonstrate the first observation of thermal diffusive dynamics for SyAFM skyrmions, which allows us to uncover the thermal effect on AF-coupled skyrmions.Ir-based SyAFM system whose ferromagnetic elements are composed of CoFeB is used for this work. The second peak of oscillation of the interlayer exchange coupling is used for maximizing antiferromagnetic exchange coupling as well as stabilizing perpendicular easy axis. Figure 1 shows the current density dependence of average velocity and SkHE for the skyrmion in the FM and SyAFM systems. We find that skyrmions in both systems are driven by current, indicating topological stabilization stemming from the skyrmion spin structure. More importantly, the skyrmions in the SyAFM system are driven by an order-of-magnitude smaller current density than those in the ferromagnetic system at comparable velocities. Based on an extended Thiele’s equation with a separately quantified SOT strength, it is shown that the achieved efficient motion of SyAFM skyrmions can be attributed to the enhanced SOTs by Pt, Ir, and W, compensated magnetic moments, and the reduction of effective topological charge. Figure 1 also shows that monotonically increasing SkHE with velocity for the FM skyrmions as in previous studies that consider an effect of pinning and/or field-like torque on SkHE3. Meanwhile, for a SyAFM skyrmion, no noticeable SkHE is observed, even in the depinning and flow regime10. Such inhibition of SkHE should be primarily caused by a reduction of the effective topological charge due to opposite magnetization at each sub-lattice. The achieved favorable properties are attributed to the employed stack structure which is engineered so that the interlayer exchange coupling, DMI, and SOT act in a concerted way10.Figure 2 shows the trajectories of SyAFM skyrmion under μ0|H| = 0.1 mT at 297.2 K measured for 1 hour. We observe that some of SyAFM skyrmions are displaced from an original position through the trajectory even though there is no bias current. The thermal diffusive dynamics of AF coupled skyrmions have been observed for the first time. We measure mean squared displacement to characterize the diffusion constant as previously conducted for FM skyrmions11. We find that the temperature dependence of diffusion constant of SyAFM skyrmions behaves approximately linear in log scale but more sensitive compared to that of FM skyrmions due to possibly suppressing gyrotropic force which stems from finite topological charge. Also, large size fluctuation of SyAFM skyrmions is observed with the thermal diffusive dynamics, which is consistent with the observed uneven sizes of skyrmions during the current induced motion. The origin could be intrinsic large thermal fluctuation of spins, relatively small interlayer exchange coupling, and/or differences in the magnetic properties between ferromagnets.In conclusion, we find that SyAFM skyrmions can be efficiently driven by currents with suppressed SkHE at room temperature, which has been a challenge of skyrmion research so far. Also, we show the first observation of the thermal diffusive dynamics of AF coupled skyrmions, which indicates that AF skyrmions would be promising to realize not only deterministic skyrmionic devices such as race-track memory12 but also unconventional computing e.g. the skyrmion-based token computing11,13.This work was supported in part by JSPS KAKENHI 19H05622. **