Magnetic skyrmions in in-plane magnets: stability, current-induced dynamics and excitations
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Abstract
The invention of transistors and microchips has revolutionized information storage. Technological progress has led to the miniaturization of microelectronics, resulting in higher energy consumption and heat production, which presents difficulties for microprocessor manufacturers. Therefore, there is a need for new computing and information technology approaches. Spintronics, which utilizes both electron spin and charge, shows great promise in overcoming the limitations of semiconductor technology and enhancing data storage capabilities. It offers increased functionality to devices and addresses current data storage constraints. An encouraging prospect among these options is the use of domain walls in racetrack memory device, enabling efficient and speedy storage of data in a non-volatile manner. Nonetheless, they encounter challenges such as the pinning of domain walls at the edges and the need for a high current density to relocate them. Skyrmionics, a new protagonist in the field of spintronics, has recently gained significant attention. Magnetic skyrmions, nanoscale windings of the spin configuration in certain magnetic materials, exhibit nontrivial topology and have the potential to replace domain walls in racetrack memories. Room-temperature observations have fueled research into skyrmion-like quasiparticles, showing lower current-driven motion (compared to domain walls) mediated by both spin-transfer and spin-orbit torques. This offers potential for racetrack memory devices, where skyrmions encode the units of information. However, the topological nature of ferromagnetic skyrmions leads to the skyrmion Hall effect, which pushes them towards the racetrack's edge, thereby causing data loss. Efficient skyrmion-based spintronic memories require the suppression of the skyrmion Hall effect and, in turn, to explore other topological spin textures. Recent studies have indicated the presence of skyrmion analogues known as in-plane skyrmions or bimerons in chiral magnet thin films with in-plane anisotropy. This thesis focuses on investigating these in-plane skyrmions in thin-film in-plane magnets. A minimal in-plane micromagnetic model is considered to assess their stability, followed by analyzing the symmetries of the Dzyaloshinskii-Moriya interaction and suggesting potential materials to host in-plane skyrmions. Furthermore, we investigate the stability of in-plane skyrmions in the monoclinic system with mirror symmetry. The thesis also explores two methods for generating in-plane skyrmions: creating magnetic bubbles through geometric constriction and releasing skyrmions from magnetic inhomogeneities. Additionally, a proof-of-concept for a racetrack utilizing in-plane skyrmions is presented. Lastly, the thesis examines the current-driven motion of in-plane skyrmions, highlighting the advantages they offer compared to Néel skyrmions through Thiele analysis and micromagnetic simulations.