Jack Harrison

Abstract:

In the quest for post-CMOS (complementary metal–oxide–semiconductor) technologies, driven by the need for improved efficiency and performance, topologically protected ferromagnetic ‘whirls’ such as skyrmions1,2,3,4,5,6,7,8 and their anti-particles have shown great promise as solitonic information carriers in racetrack memory-in-logic or neuromorphic devices1,9,10,11. However, the presence of dipolar fields in ferromagnets, which restricts the formation of ultrasmall topological textures3,6,8,9,12, and the deleterious skyrmion Hall effect, when skyrmions are driven by spin torques9,10,12, have thus far inhibited their practical implementation. Antiferromagnetic analogues, which are predicted to demonstrate relativistic dynamics, fast deflection-free motion and size scaling, have recently become the subject of intense focus9,13,14,15,16,17,18,19, but they have yet to be experimentally demonstrated in natural antiferromagnetic systems. Here we realize a family of topological antiferromagnetic spin textures in α-Fe2O3—an Earth-abundant oxide insulator—capped with a platinum overlayer. By exploiting a first-order analogue of the Kibble–Zurek mechanism20,21, we stabilize exotic merons and antimerons (half-skyrmions)8 and their pairs (bimerons)16,22, which can be erased by magnetic fields and regenerated by temperature cycling. These structures have characteristic sizes of the order of 100 nanometres and can be chemically controlled via precise tuning of the exchange and anisotropy, with pathways through which further scaling may be achieved. Driven by current-based spin torques from the heavy-metal overlayer, some of these antiferromagnetic textures could emerge as prime candidates for low-energy antiferromagnetic spintronics at room temperature1,9,10,11,23.

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.