Nanoscale Scanning Diamond Magnetometry of Antiferromagnets

Antiferromagnetic materials are receiving widespread attention because they promise to enable smaller, faster, and more resilient data processing units compared to state-of- the-art ferromagnets. The possibility to control the orientation of the magnetic domains in antiferromagnets by either electrical or optical means is revolutionizing the field of magnetism and opening a wealth of applications in spintronics. Outstanding open questions bear on both fundamental problems (switching mechanism) as well as on applied aspects (amplitude of the readout signal). One of the great unknowns in the field of antiferromagnetism is the structure of domain walls, which is key to both of these aspects. In particular, chiral walls are required for current-controlled magnetisation switching. The limited knowledge about antiferromagnetic domain walls as well as the switching mechanism is mainly due to the difficulty of imaging antiferromagnets. The main focus of this thesis is to address the aforementioned problems by imaging antiferromagnets using nanoscale scanning diamond magnetometry. We will elaborate throughout this thesis on the question of what information about the spin structure can be inferred when measuring the magnetic stray field on the surface of an antiferromagnet. Related to that, we discuss current-induced effects in antiferromagnets and to what extent those can be detected using nanoscale diamond magnetometry. We first calibrate our technique by determining the internal structure of a domain wall in a ferrimagnetic insulator Tm3Fe5O12 and estimate its magnetisation. Ferrimagnets, being in a broader sense between ferro- and antiferromagnets, are detectable by common magnetic probes, which we use to confirm our methodology. We reveal that the domain walls in Tm3Fe5O12 have a left Néel character indicating the presence of interfacial Dzyaloshinskii-Moriya interaction. These results open the possibility to stabilize chiral spin textures in centrosymmetric magnetic insulators important for spintronic applications. We then extend these insights and methods to the archetypical antiferromagnet Cr2O3. Here we present, to the best of our knowledge, the first experimental observation of a domain wall in a pure, intrinsic antiferromagnet. We reveal that the intrinsic domain wall structure is Bloch-like and can become Néel-like if sufficient in-plane magnetic anisotropy is present. Our experimental observation is significant because the theory does not predict any preference for Bloch or Néel domain wall structure. Finally, we combine nanoscale scanning diamond magnetometry with electrical pulsing and resistance measurements to shed light on the switching mechanism and readout signal in the metallic antiferromagnet CuMnAs. We find that, besides the known reorientation of the Néel vector, the domain pattern fragments upon injection of current pulses. This provides an explanation for the recently-observed unipolar high-resistive switching signals in CuMnAs, and their relaxation and demonstrates a novel memristive effect. Besides these observations, we show that nanoscale scanning diamond magnetometry can be successfully applied to map the domain structure of in-plane antiferromagnets. Our results point out directions for future research in the field of antiferromagnetic spintronics and provide a new methodology and concepts relevant for the quantum sensing and imaging communities.