STABILISATION, MANIPULATION AND EVOLUTION OF MAGNETIC SKYRMIONS IN THIN FILMS AND NANOSTRUCTURES

Abstract

Magnetic skyrmions are topologically non-trivial spin textures enabling the development of next-generation spintronic data storage devices with low power consumption, high data density and non-volatility. The recent discovery of room-temperature skyrmions in Co or CoFeB-based multilayer systems has offered a high degree of freedom in tunability in magnetic properties. In these systems, magnetic interactions can be tuned via various surface or interfacial engineering methods, resulting in a modulation of skyrmion properties such as variation in size and enhanced thermal stability. One of the most profound interactions for the stabilisation of skyrmions is the interfacial Dzyaloshinskii-Moriya interaction (DMI) arising from the inversion asymmetry at the interface with the presence of the spin orbit coupling (SOC). However, quantification of the DMI in multilayer systems is always challenging. This thesis provides an examination of the interfacial DMI in Pt/Co/Ta multilayers using a stripe domain model and shows a successful approach to stabilise zero-field skyrmion. All Pt/Co/Ta multilayers were deposited using a magnetron sputtering system. A combination of characterisation techniques including X-ray reflectivity (XRR), vibrating sample magnetometer (VSM) and magnetic force microscopy (MFM) were used to study the structural and magnetic properties of Pt/Co/Ta multilayers. The first part of the work in this thesis explores the change of various magnetic properties as a function of the Co layer thickness. DMI is quantified by using static magnetic domain images with a stripe domain model. The experimental results eventually clarify the applicability of the model and show a well-defined linear behaviour for samples with quality factor Q > 1.2. Additionally, micromagnetic simulations were performed to evaluate the effect of each magnetic parameter, providing a guidance on engineering Co-based multilayers with the desired properties for skyrmion applications. The second part of the work demonstrates the stabilisation of remanent skyrmions in confined nanostructures via tilted saturation fields. The skyrmion profile was investigated using magnetic force microscopy (MFM). By tilting the magnetic field from the in-plane axis to the out-of-plane axis, magnetic states are transformed from stripe domain dominated states to ferromagnetic states, passing through skyrmion dominated states. This phenomenon is then revealed by micromagnetic simulations. Moreover, the generation of skyrmions using a magnetic tip has been explored. These results demonstrate two zero-field skyrmion generation methods, and shedding light on future developments for skyrmion-based nanodevices.

Date of Award	13 Nov 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Thomas Thomson (Co Supervisor) & Christoforos Moutafis (Main Supervisor)