Magnetic skyrmions in synthetic antiferromagnets: stability, dynamics, and self-propulsion

Abstract

Magnetic skyrmions are magnetic textures resembling swirls of magnetization stabilized in ferromagnetic (FM) materials and with growing interest for spintronics applications given their inherent topological stability, nanoscale size, and current-induced motion using very low threshold currents. This thesis aims to investigate the stability, current-induced dynamics, and emergent self-propulsion of these topological textures in synthetic antiferromagnets (SAFs) — a multilayer system based on antiferromagnetically coupled FM layers. For that, we adopt a unified methodology combining micromagnetic simulations of the semiclassical stochastic Landau-Lifshitz-Gilbert (LLG) equation with analytical models based on the quasi-particle description provided by the Thiele equation. Initially, we describe the magnetic phase diagram found for homochiral and heterochiral SAFs — respectively composed by FM layers with parallel and antiparallel DMI vectors — where we focus in the coaxial and non-coaxial skyrmionskyrmion bound pairs obtained during our study. Particularly, we verify that the increase of the interlayer Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling enables the non-coaxial pair as the preferred bound state of the heterochiral SAF. Then we proceed to investigate the pair dynamics in unbalanced SAFs, featuring asymmetries in the Dzyaloshinskii-Moriya interaction (DMI) and in the magnetic saturation of the FM layers. SAFs are known to provide a vanishing net topological charge and, consequently, the cancellation of the Skyrmion Hall effect (SkHE)—the transverse deflection of skyrmions in motion in a FM layer. The elimination of SkHE is crucial for spintronics applications envisioning the skyrmion as the information carrier, such as the case of racetrack memories. By investigating the persistence of SkHE when skyrmions are set into motion by using the Zhang Li model for the spin-transfer torque (STT) effect, we demonstrate that the cancellation of the SkHE is robust against imbalances in several material parameters such as the relative magnitudes of DMI and anisotropy, but sensitive to asymmetries in the injection of spin-polarized currents into the FM layers and in the magnetic saturations. Furthermore, we reveal that heterochiral SAFs not only stabilize bound pairs in a non-coaxial configuration but also facilitate their experimental detection via stray fields. In a second phase of our study, we theoretically demonstrate that these non-coaxial pairs feature a remarkable behavior, functioning as nanoscale self-propelled, autonomous “swimmers”. By exciting symmetric (SBM) or antisymmetric (ABM) breathing modes through oscillating external electric or magnetic fields, the time-reversal symmetry of the pair is broken, generating non-reciprocal gyrotropic (Magnus) forces that result in a net displacement of the pair, with the selfpropulsion velocity oriented perpendicularly to the bond direction. The Thiele-based model fully describes the symmetric excitation at weak interlayer coupling and allows for the calculation of the self-propulsion velocity by projecting the skyrmion trajectory onto the shape space spanned by the bond length of the skyrmion pair and the dissipation coefficient. The self-propulsion is found to be maximized when the excitation frequency matches the natural frequency of the breathing modes and reaches ultra-high speeds, in the order of hundreds of millions of body lengths per second, more than any known natural or artificial swimmer. Finally, the stochastic analysis further reveals that the thermal noise induces the skyrmion pair to behave as an active Brownian particle (ABP), featuring transition from diffusion-dominated to ballistic motion when the excitation amplitude is increased. The increase of temperature makes the skyrmion pair to exhibit spontaneous reorientations and momentary reversals of the propulsion, behaviors akin to biological swimmers (e.g., flagellated bacteria and microalgae). Although the swimming mechanism for the case of strong coupling is not yet fully understood, our results substantiate the proposal of the skyrmionic active matter based on the motion of skyrmion-skyrmion bound pairs in heterochiral SAFs, opening newperspectives for the design of reconfigurable magnetic metamaterials and neuromorphic computing devices based on these new autonomous topological textures, ultimately bridging the gap between spintronics and active matter physics.