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RESEARCH PRODUCT

Skyrmion Dynamics – from thermal diffusion to ultra-fast motion

subject

description

Summary form only given. Spintronics promises to be a paradigm shift from using the charge degree of freedom to using the spin degree of freedom. To this end three key requirements are: (i) stable spin structures for long term data retention; (ii) efficient spin manipulation for low power devices and (iii) ideally no susceptibility to stray fields as realized for antiferromagnets. We explore different materials classes to tackle these challenges and explore the science necessary for a disruptive new technology. To obtain ultimate stability, topological spin structures that emerge due to the Dzyaloshinskii-Moriya interaction (DMI), such as chiral domain walls and skyrmions are used. These possess a high stability and are of key importance for magnetic memories and logic devices [1,2]. We have investigated in detail the dynamics of topological spin structures, such as chiral domain walls that we can move synchronously with field pulses [3]. We determine in tailored multilayers the DMI [4], which leads to perfectly chiral spin structures. For ultimately efficient spin manipulation, spin transfer torques are maximized by using highly spin-polarized ferromagnetic materials that we develop and we characterize the spin transport using THz spectroscopy [2]. Furthermore, we use spin-orbit torques, that can transfer 10x more angular momentum than conventional spin transfer torques [4-6]. We then combine materials with strong spin-orbit torques and strong DMI where novel topologically stabilized skyrmion spin structure emerge [5]. Using spin-orbit torques we demonstrate in optimized low pinning materials for the first time that we can move a train of skyrmions in a "racetrack" -type device [1] reliably [5,6]. We find that skyrrnions exhibit a skyrmion Hall effect leading to a component of the displacement perpendicular to the current flow [6]. We study the field-induced dynamics of skyrrnions [7] and find that the trajectory of the skyrmion's position is accurately described by our quasi particle equation of motion. From a fit we are able to deduce the inertial mass of the skyrmion and fi nd it to be much larger than inertia found in any other magnetic system, which can be attributed to the non-trivial topology [7]. While thus highly reproducible driven skyrmion motion is possible, we have recently developed new ultra-low pinning multilayer stacks, which exhibit thermally activated dynamics of skyrmions. Here the energy landscape is sufficiently fl at so that we observe pure diffusive motion of skyrmion quasiparticles. In contrast to predictions where diffusion was expected to be largely suppressed [8], we find that skyrmions exhibit diffusion at a range of temperatures. Furthermore, in contrast to the analytical calculations, we fi nd a strong temperature and size dependence of the diffusion and we can explain these observations based on thermally activated excitations of th e skyrmions. By varying the temperature and drive, we finally probe the transition from thermally activated diffusion to the viscous flow regime for the first time and quantify the skyrrnion Hall angle across the full dynamics range.