title: Manipulation and exploitation of the dynamic processes of Skyrmions
Creators: name: Rendell-Bhatti, Frederic
Publisher: name: University of Glasgow/Queen's University Belfast
Year: 2022

description: Magnetic skyrmions are emergent, topological quasiparticles with vortex-like magnetisation profiles, stabilised in magnetic systems hosting the Dzyaloshinskii–Moriya (DM) interaction. Their high physical stability and ease of propagation by electrical currents, means that skyrmions have the potential to act as information carriers in future low-power data storage and transmission devices. However, for skyrmions to be used in such a manner, their stability under non-equilibrium conditions and in a range of magnetic systems must be investigated further. This thesis provides a contribution to such studies, through investigating the dynamics and physical stability of magnetic skyrmions in a range of material systems. Arrays of skyrmions may be stabilised into a hexagonal skyrmion crystal lattice (SkX), under favourable conditions of magnetic field and temperature. Image analysis was performed on high frame rate Fresnel LTEM video data, showing repeated, spontaneous skyrmion motion across a SkX domain boundary. It was observed that the motion involved the creation and annihilation of skyrmions through splitting and merging of deformed skyrmions. The energy landscape of a SkX domain boundary region was investigated using micromagnetic simulations, with SkX defects found to be dominated by the interplay between exchange and Zeeman energies. Informed by Fresnel LTEM imaging and micromagnetic simulations, a mechanism for skyrmion creation and annihilation involving anti-skyrmions is proposed. This work demonstrates that in regions of high energy density, skyrmions may exhibit such large deformations that they are able to spontaneously split or merge with neighbouring skyrmions. These observations highlight the limits of skyrmion stability when alternative energy pathways involving topological structures are available. Advanced image processing including total variation (TV) denoising and nonnegative matrix factorisation (NMF) was carried out on sub-millisecond Fresnel image frames showing skyrmion dynamics, providing the ability to automatically identify transient skyrmion states. These analysis techniques provide a means of improving signal-to-noise ratio (SNR) and configurational state identification using machine learning. Perturbation of the SkX is carried out through the application of sub-µs perpendicular magnetic field pulses within the TEM. The realisation of such in-situ magnetic field pulses was achieved through the design, creation and testing of a bespoke microcoil and current pulser. Realisation of extensive disorder, and the formation of many defect regions within the SkX is demonstrated, where long-range reorientation of the SkX is unable to occur due to the duration of the pulses. Application of precise magnetic field pulses will allow for greater control over skyrmion nucleation and may aid in the study of skyrmion metastability. Methods for enhancing skyrmion stability in the technologically relevant synthetic antiferromagnetic (SAF) systems are investigated through the utilisation of micromagnetic simulations. Variation in skyrmion stability within multilayer SAF systems is described, through studying the process of offsetting the destabilising dipole interactions with interlayer exchange coupling. SAF systems with ’spinterface’ regions are investigated, focusing on the skyrmion stabilising effect of including a C60–metal interface. Non-trivial skyrmion size behaviour is shown when the C60 is explicitly simulated as a dilute magnetic layer, due to coupling between three distinct magnetic subsystems. These micromagnetic studies highlight the ability to tune magnetic interactions, and thus skyrmion stability, through the engineering of multilayer condensed matter systems.