Exploring antiferromagnetic domain wall mechanics through scanning nitrogen vacancy magnetometry

As computing requirements and data volumes continue to increase, the need for faster, more efficient memories has become a driving force in many research areas. One proposed alternative to current ferromagnetic (FM) storage technology is antiferromagnetic (AFM) memories, which promise faster, more energy-efficient switching and higher bit densities. Developing such technologies requires progress on two fronts. On the one hand, we must understand and harness the AFM magnetic textures central to the proposed memory devices. At the same time, we require technologies capable of addressing these typically hard-to-access systems. In this thesis, we use magnetometry based on the nitrogen vacancy (NV) center in diamond to address both sides of this problem.
NV magnetometry, in particular scanning magnetometry, can be employed in a variety of environmental conditions, providing access to a wide range of materials and phenomena. Furthermore, the high magnetic field sensitivity and spatial resolution achievable with this technique enable us to address the nanoscale magnetic textures of interest. However, sensitivity and resolution are limited by our ability to collect the NV center photoluminescence (PL) and by how close we can bring the NV to the magnetic field source, respectively. Here, we aim to improve on the state-of-the-art scanning NV magnetometry probe with a novel design based on a truncated parabolic pillar. The parabolic nature of the pillar leads to excellent directional emission of the PL, allowing us to demonstrate median PL rates of 2.1 MHz and collection efficiencies of 57%. As such, we realize improved sensitivities compared to the state-of-the art scanning probes while simultaneously achieving nanoscale resolution through the truncated end facet of the pillar.
We then use these improved scanning probes to study the magnetic properties of a magnetoelectric AFM: chromia. Due to its room-temperature AFM ordering and the ability to switch the magnetic order with electric fields, this material is a popular candidate for spintronics applications. In our study, we demonstrate control over the magnetic orientation of a bulk chromia (Cr$_2$O$_3$) crystal and employ its magnetoelectric properties to nucleate domain walls (DWs). Using NV magnetometry, we characterize the surface magnetization of Cr$_2$O$_3$ and investigate the DW structure. We furthermore demonstrate an interaction between the DW and patterned surface topography, allowing us to develop a model of the DW mechanics. In particular, we observe a Snell's law-like behavior of the DW in the presence of topographical steps and pinning of the DW to the edges of these steps. We use this pinning, together with local heating of the Cr$_2$O$_3$ crystal, to exert control over the motion of the DW.
These results bring us one step closer to achieving AFM-based memories. Having shown the ability to generate, control, and move DWs in an AFM crystal, we have laid the groundwork for a DW-based memory. Moreover, by improving the understanding of antiferromagnetic DW mechanics, we highlight material properties of Cr$_2$O$_3$ that may benefit and guide future material research.