A Lorentz microscopy study of chiral magnetic textures stabilised in thin films by an interfacial Dzyaloshinskii-Moriya interaction

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)

Quantitative Differential Phase Contrast Imaging of the Magnetostructural Transition and Current-driven Motion of Domain Walls in FeRh Thin Films

No abstract available

Model-based iterative reconstruction of three-dimensional magnetization in a nanowire structure using electron holographic vector field tomography

Experimental techniques for the characterization of three-dimensional (3D) magnetic spin structures are required to advance the performance of nanoscale magnetic technologies. However, as component dimensions approach the nanometer range, it becomes ever more challenging to analyze 3D magnetic configurations quantitatively with the required spatial resolution and sensitivity. Here, we use off-axis electron holography and model-based iterative reconstruction to reconstruct the 3D magnetization distribution in an exemplary nanostructure comprising an L-shaped ferromagnetic cobalt nanowire fabricated using focused electron beam induced deposition. Our approach involves using off-axis electron holography to record tomographic tilt series of electron holograms, which are analyzed to reconstruct electron optical magnetic phase shifts about two axes with tilts of up to ±60◦. A 3D magnetization vector field that provides the best fit to the tomographic phase measurements is then reconstructed, revealing multiple magnetic domains in the nanowire. The reconstructed magnetization is shown to be accurate for magnetic domains that are larger than approximately 50 nm. Higher spatial resolution and improved signal-to-noise can be achieved in the future by using more specialized electron microscopes, improved reconstruction algorithms, and automation of data acquisition and analysis

Curved nanomagnets: an archetype for the skyrmionic states at ambient conditions

Stabilizing magnetic skyrmions is a critical issue in spintronics, impacting data storage and computing. This study investigates skyrmion and skyrmionium phenomena within a hexagonal array of curved nanomagnets. Utilizing atomistic calculations, micromagnetic simulations, and experimental methods such as magnetic force microscopy and electron holography, we analyze the interplay between magnetic parameters, curvature, and the interfacial Dzyaloshinskii–Moriya interaction (iDMI) in the formation of these structures. We observed that isolated skyrmions and mixed skyrmionic phases can spontaneously form in a symmetric Pt/Co/Pt multilayer curved nanomagnet matrix without external fields at room temperature. Our findings highlight the considerable influence of geometric curvature on iDMI, providing insights for engineering skyrmionic configurations. This research enhances our understanding of nanomagnetism and contributes to the advancement of skyrmion-based technologies

Phase coexistence and transitions between antiferromagnetic and ferromagnetic states in a synthetic antiferromagnet

In synthetic antiferromagnets (SAFs), antiferromagnetic (AFM) order and synthesis using conventional sputtering techniques is combined to produce systems that are advantageous for spintronics applications. Here we present the preparation and study of SAF multilayers possessing both perpendicular magnetic anisotropy and the Dzyaloshinskii-Moriya interaction. The multilayers have an antiferromagnetically aligned ground state but can be forced into a full ferromagnetic (FM) alignment by applying an out-of-plane field ∼100 mT. We study the spin textures in these multilayers in their ground state as well as around the transition point between the AFM and FM states at fields ∼ 40 mT by imaging the spin textures using complementary methods: photoemission electron, magnetic force, and Lorentz transmission electron microscopies. The transformation into a FM state by field proceeds by a nucleation and growth process, where skyrmionic nuclei form and then broaden into regions containing a ferromagnetically aligned labyrinth pattern that eventually occupies the whole film. Remarkably, this process occurs without any significant change in the net magnetic moment of the multilayer. The mix of antiferromagnetically and ferromagnetically aligned regions on the micron scale in the middle of this transition is reminiscent of a first-order phase transition that exhibits phase coexistence. These results are important for guiding the design of spintronic devices whose operation is based on spin textures in perpendicularly magnetized SAFs

Model-Based Iterative Reconstruction of Three-Dimensional Magnetisation in a Nanowire Structure Using Electron Holographic Vector Field Tomography

Reconstructed phase images and Python code for 3D reconstruction, simulation, and image processing

Characterisation of Skyrmion Spin Textures in CoB/CoFeB Multilayers

No abstract available