title: A Lorentz microscopy study of chiral magnetic textures stabilised in thin films by an interfacial Dzyaloshinskii-Moriya interaction
Creators: name: Fallon, Kayla
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: chiral magnetism, magnetism of thin films, skyrmions, DMI, Lorentz microscopy, TEM

description: The Dzyaloshinskii-Moriya interaction (DMI) is an antisymmetric exchange interac- tion that arises at interfaces between ferromagnets and heavy metals which possess strong spin-orbit coupling. Interface-driven DMI promotes Néel type magnetic textures with a fixed chirality, including skyrmions: particle-like magnetic objects. These chiral magnetic structures have promising properties for applications in spintronic devices. For skyrmions, these favourable properties include their small size, their fast and efficient motion under spin-polarised currents and the possibility of electrical detection. This thesis presents a number of studies on the various effects of DMI on the magnetic textures stabilised in thin films using primarily the methods of Lorentz microscopy. The contrast expected in Lorentz microscopy from simple Néel and Bloch magnetic objects is outlined, and a theoretical method of accessing contrast directly related to the in-plane magnetisation of Néel type magnetic objects (which is not generally accessible in Lorentz microscopy) is proposed. This framework is then expanded upon to quantify ‘hybrid chiral’ wall structures that can be stabilised in multilayers where the DMI energy and dipolar energy are similar orders of magnitude. The presence and extent of the hybrid structure is assessed for three distinct multilayered systems and identifies a Bloch twist, indicative of hybrid chirality, in multilayers comprised of 10 and 15 repeats but not in a multilayer with five repeats. This information is critical in permitting an informed choice on the spin-injection geometry best suited for motion of the skyrmions. Field-induced skyrmion nucleation at artificial nanoscale defects, created in a controlled and repeatable manner with focused ion beam (FIB) irradiation, was studied using Lorentz microscopy and correlated to structural information gained from standard transmission electron microscopy (TEM) images. It was found that this nucleation method has three notable advantages: (i) controlled localisation of nucleation; (ii) stability over a larger range of external field strengths, including stability at zero field; and (iii) existence of skyrmions in material systems where, prior to defect fabrication, skyrmions were not previously obtained by field-cycling. Additionally, it is observed that the size of defect nucleated skyrmions appears to be uninfluenced by the defect itself. All of these characteristics are expected to be useful towards the goal of realising a skyrmion-based spintronic device. Finally the effects of DMI on magnetic vortices in planarly magnetised films are studied using micromagnetic simulations and Lorentz microscopy. Micromagnetic simulations predict that there is a DMI-dependent chiral twist (best quantified as divergence) of the magnetisation about the vortex core. Using Lorentz microscopy this effect is measured in two ways and, if attributable to DMI, the DMI strength is estimated to be |D| ≈ 1 mJm^(−2).