DescriptionAntiferromagnetic order in topological materials has recently become incredibly important in the quest for exotic phenomena such as high temperature quantum anomalous Hall effect, quantized magnetoelectric effect, chiral edge modes, and Weyl and Dirac semimetal states. However, there is a lack of understanding of the antiferromagnetic domains and domain walls in these materials. The control of these domain walls or spin textures is not only important for controlling these states, but also crucial for spintronic applications of antiferromagnets. Despite many efforts, it has been challenging to directly visualize antiferromagnetic domains or domain walls with nanoscale resolution, especially in magnetic field.
In this thesis, we show magnetic imaging of domain walls in several uniaxial antiferromagnets including the topological insulator MnBi$_2$Te$_4$ family and the Dirac semimetal EuMnBi$_2$, using cryogenic magnetic force microscopy (MFM). Our MFM results reveal higher magnetic susceptibility inside the domain walls than in domains. Domain walls in these antiferromagnets form randomly with strong thermal and magnetic field dependence.
Through this application, we also present microscopic evidence of the persistence of uniaxial A-type antiferromagnetic order to the surface layers of MnBi$_2$Te$_4$ single crystals using magnetic force microscopy. Our results reveal termination-dependent magnetic contrast across both surface step edges and domain walls, which can be screened by thin layers of soft magnetism. The robust surface A-type order is further corroborated by the observation of termination-dependent surface spin-flop transitions, which have been theoretically proposed decades ago and not observed in natural antiferromagnets until now.
The direct visualization of these domain walls and domain structures in magnetic field not only provides key ingredients for understanding the electronic properties of the antiferromagnetic topological insulator MnBi$_2$Te$_4$, but also opens both a new way of exploring intrinsic surface metamagnetic transitions in natural antiferromagnets and a new path toward control and manipulation of domain walls or spin textures in functional antiferromagnets.