Jack Harrison

Abstract:

Whirling magnetic textures can have topological properties, enhancing their stability over and above that derived from energetic considerations. Such structures have been proposed as data carriers in next-generation post-Moore computing. Whilst abundantly observed in ferromagnets, their antiferromagnetic counterparts are more elusive. Interest in antiferromagnetic topological textures for device applications is growing, due to their predicted ultra-fast, deflection-free dynamics whilst being robust against external fields. In this thesis, I develop processes for imaging, nucleating and controlling topological textures in antiferromagnets, targeted towards their integration in next-generation racetrack-based oxide electronics. The prototypical canted antiferromagnet α-Fe2O3 is used throughout as an interesting test case, due to the family of topological textures present at room temperature that can be repeatedly nucleated via a Kibble-Zurek-like quench.

I developed analytical and micromagnetic models for topological textures in A-type antiferromagnets, focusing on the scaling of textures with relevant material parameters, allowing us to push towards the ultra-small sizes relevant for device applications. This was also used to predict the existence of the long sought-after topological antiferromagnetic skyrmions. I investigated freestanding crystalline α-Fe2O3 nanomembranes, a novel form of matter developed by my collaborators. One key conclusion of these experiments was that defects strongly affect the first-order Morin transition, whilst maintaining the Kibble-Zurek phenomenology observed in thin films attached to substrates. Magnetic fields cause domain repopulation in this canted AFM, but topological textures were observed to be stable in the presence of moderate field perturbations. Finally, freestanding crystal membranes can host relatively large strains compared to attached thin films or bulk crystals, which have similar lateral dimensions but the latter are drastically thicker. This was used to produce an athermal route to nucleate topological textures and tune domain populations, opening novel pathways for exploring Kibble-Zurek phenomenology in crystal membranes, as well as providing an interesting route towards device applications.