  The control of domain walls or spin textures is 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. Here, we report magnetic imaging of
domain walls in several uniaxial antiferromagnets, 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 or net moments inside the domain walls than in domains. Domain
walls in these antiferromagnets form randomly with strong thermal and magnetic
field dependences. The direct visualization of domain walls and domain
structure in magnetic field will not only facilitate the exploration of
intrinsic phenomena in topological antiferromagnets, but also open a new path
toward control and manipulation of domain walls or spin textures in functional
antiferromagnets.
