Creation and manipulation of topological magnetic textures in chiral and frustrated magnets
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Abstract
Magnetic skyrmions are vortex-like spin textures in ferromagnetic materials that have seen a surge in research interest in recent years. Their small size, energetic stability, and the low applied electric currents required for their manipulation have made them promising candidates in next-generation information storage and processing applications. Skyrmions are a subset of a broader class of topological magnetic textures which includes domain walls and more novel structures such as magnetic hopfions. Magnetic hopfions are three-dimensional, smoke ring-like topological magnetic structures which are experiencing a steady growth in research interest at the time of preparing this Thesis.
In this Thesis, we investigate the creation and dynamics of magnetic solitons, with a particular focus on magnetic skyrmions, subject to various external influences. The Thesis is divided into three parts.
In the first Part, we review relevant concepts, methods, and literature. Specifically, we introduce magnetic skyrmions and hopfions through topological considerations, as well as the mechanisms through which they are stabilised. We also discuss the micromagnetic model, which is the framework within which we perform both the analytical and numerical calculations.
In the second Part of this Thesis, we investigate the current-driven creation of skyrmions and vortex rings in chiral magnets. As electrons pass through a magnetic material, their spin aligns with the local magnetisation, which induces a torque that acts on the magnetisation. This torque is known as the spin-transfer torque and causes the magnetisation to vary over time. The interplay between spin-transfer torque and magnetic impurities can result in the creation of magnetic textures at the impurity. Following from earlier literature, we propose a method whereby skyrmions can be created in spin spiral states. Furthermore, we demonstrate a similar mechanism through which vortex rings can be created in bulk magnetic systems.
In the final Part of this Thesis, we study the dynamics of skyrmions in both chiral and frustrated magnets. In the former case, we show that subjecting skyrmions to an oscillating magnetic field at unit fractions of their eigenfrequencies can result in efficient excitation of the eigenmodes. This has applications in the frequency multiplication of magnons. In the latter case, we show that the application of an applied electric field can excite internal modes of the skyrmion, which we describe using the topology of magnetic hopfions.