Probing the Nature of the Magnetic Interactions in Magnetic Skyrmions

Abstract

Magnetic skyrmions are topologically-protected, solitonic entities which are composed of a vortex-like arrangement of magnetic moments. Recently, they have seen a surge of interest from the condensed matter physics community due
to their presence in a number of systems which feature novel and interesting physical phenomena, such as unique transport and frequency responses. These characteristics, together with their countable nature is highly appealing for use
within novel spintronic devices. With the ever-growing demand for high processing speeds and data storage, skyrmionic devices have the potential to provide
low-power, high density data storage as well as play a key role in a number of next-generation computational devices. Whilst skyrmions do appear in a number of different systems, in this thesis we will focus on skyrmions within bulk-magnets. In these bulk-systems, skyrmions are stabilised within a small range of temperatures and applied magnetic fields. Commonly, skyrmions are shown to be stabilised by the interplay of the exchange, Dzyaloshinskii-Moriya and Zeeman energies, together with thermal fluctuations or higher-order magnetic anisotropy. Typically, these interactions are only modelled to first order, but there is increasing interest in higher-order and alternative interactions to both generate and manipulate skyrmions.
In this thesis, at Chapter 1 we start with explaining why it is the material specific interactions, and not the particular arrangement of spins, which give rises
to the skyrmion stability. We investigate real material parameters and the spintextures those interactions stabilise using diffraction-based techniques, which we
give a brief overview in Chapter 2. In Chapter 3, we design and perform an experiment for measuring and quantifying the anisotropic exchange interaction, and show that this often overlooked interaction is crucial for the formation of
the recently discovered tilted conical phase. In Chapter 4, we look at how to control this tilted conical phase using the magnetoelectric coupling interaction,
and show that we are reliably able to manipulate the conical wavevector into different positions via an electric field, and provide mean-field theory to explain
the tilt-directions as a function of electric field. Furthermore, we provide micromagnetic simulations of a toy model to show the potential spintronic applications
of this manoeuvrability. In Chapter 5, we move away from chiral magnet systems to investigate the spin-textures within a nanoskyrmion-containing material.
We find clear phase boundaries between a number of incommensurate magnetic phases, and perform an experiment to accurately resolve their form. We find interesting behaviour with regards to their chirality, which we suggest to arise
from local Dzyaloshinskii-Moriya interactions despite the overall centrosymmetric crystal structure. In Chapter 6, we return to chiral magnets and experiment with metastable skyrmions created using a custom fast field cooling method. We then discover a remarkable phenomenon that the skyrmion intensity grows as a function of field, which is contrary to previous literature suggesting that their
stability massively decreases. We suggest that both anisotropic interactions and disorder play a role, and perform a thorough directionally dependent study to try
to decouple the two arguments. Finally in Chapter 7, we provide the conclusions of this thesis.