Skyrmions in motion – thermal dynamics and ordering in 2D
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Abstract
Magnetic skyrmions are chiral magnetic spin textures of non-trivial topology, whose topological stabilization allows them to be treated as quasi-particles.
Owing to their nanoscale size and efficient means of manipulation, skyrmions have attracted considerable research interest as potential information carriers in future low-power data storage and data processing devices. However, reliable device operation requires strict conditions on performance, thermal stability, and robustness against external perturbations, which remain challenging to achieve. Consequently, exploring the static and dynamic properties of skyrmions, as well as their interactions with each other and the host material, is an ongoing and central topic of spintronics research.
A major factor limiting skyrmion dynamics is pinning, caused by material inhomogeneities that induce non-uniform magnetic properties. In this thesis, I show that skyrmions in CoFeB thin-film multilayer stacks experience a continuous two-dimensional energy landscape, leading to pronounced pinning effects. Pinning is dominated by the skyrmion boundary, implying that skyrmions cannot be fully described as point particles, but that their finite size and shape play a decisive role. Consequently, the pinning strength depends sensitively on skyrmion size. Since the skyrmion size can be tuned efficiently by magnetic field excitations, oscillating fields periodically modulate the skyrmion radius, thereby reducing effective pinning and enabling depinning. Under such conditions, the diffusion coefficient increases by up to two orders of magnitude compared to thermal diffusion without external driving.
The chosen magnetic CoFeB multilayer composition stabilizes skyrmions of approximately 1 µm diameter near room temperature, allowing for direct optical imaging with Kerr microscopy. This provides unique access to the thermal dynamics of individual skyrmions and their collective behavior. Beyond their relevance for spintronic applications, such micrometer-sized skyrmions in nanometer-thin films constitute an ideal model system to study fundamental two-dimensional physics. In particular, dense skyrmion arrangements exhibit Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) phase behavior and the corresponding phase transitions. I demonstrate that skyrmion lattices can form spontaneously in these systems, although the non-flat energy landscape prevents the emergence of true quasi-long-range order. Instead, multiple lattice domains of distinct orientation develop, separated by effectively pinned domain boundaries that impose boundary conditions on the surrounding lattice.
By confining skyrmion lattices to finite geometries, such boundary conditions can also be imposed artificially. I show that commensurate hexagonal confinement enhances lattice order and favors uniform orientation, whereas non-commensurate geometries lead to multi-domain configurations with suppressed order. Furthermore, I show that skyrmion lattices in a solid regime with translational order can be melted into a disordered, isotropic liquid regime through an intermediate hexatic phase exhibiting only orientational order, in agreement with KTHNY theory. The melting transitions can be driven both by shrinking skyrmions, which reduces the packing fraction, and by applying oscillating fields, which enhance diffusive dynamics. Crucially, direct time-resolved imaging enables the identification and tracking of the topological defect dynamics that mediate the melting transitions -- the key feature of KTHNY theory.
In summary, this thesis presents experimental studies of thermally diffusing magnetic skyrmions, revealing how they interact with the underlying magnetic energy landscape and how their diffusion can be tuned by external fields. I demonstrate how skyrmion lattices form, respond to geometric confinement, and undergo phase transitions, thereby establishing skyrmions as a unique platform for investigating two-dimensional melting and related fundamental phenomena with unprecedented resolution.