Experimental access to mesoscale structures of antiferromagnetic (AF) materials is of vital importance with rapidly growing demands from both fundamental physics investigations as well as technological applications. [1][2][3][4][5][6] Many complex oxides, which typically have AF orders in their Mott insulating phases, have the innate propensity toward electronic phase separations upon chemical doping, and exotic physical properties arise out of nanoscale emulsions of different electronic orders. For example, high-temperature superconductivity in cuprates emerges from inhomogeneous spin and charge densities, [7,8] and colossal magnetoresistivity in manganites arise from a percolative transport through ferromagnetic domains separated by chargeordered insulating regions. [9,10] For parent AF phases, information on the domain wall structures are of technological importance in the field of spintronics, as they offer enhanced stability and unprecedented velocity controlled by spin-orbit torques. [11,12] Measurements on a mesoscale length scale, however, is challenging for several reasons. First, the detection of an AF order itself is difficult due to the net cancellation of the magnetic moment. A variety of experimental techniques have been developed for imaging of AF materials but only few of them couple directly to the AF order (see Section SI, Supporting Information, and ref. [13]). Second, there is a trade-off between spatial resolution and field of view (FOV). While scanning-type microscopes, based on spin-polarized tunneling, [14] magnetic force, [15] or nitrogen-vacancy defects in diamond, [16] provide atomicscale resolutions, their narrow FOV limit studies of mesoscale structures. Third, the experimental condition can be highly restrictive. For instance, photoelectron emission microscopy [17] requires a clean, flat sample surface prepared in ultrahigh vacuum, and an external magnetic field cannot be applied.X-ray-based imaging techniques have particular advantages in studying mesoscopic structures in buried interfaces, or those in complex environments. For instance, diffraction-based fullfield imaging technique has been successfully applied to probe the strain and defects in single-and polycrystals, [18,19] domain distribution and its evolution in thin films, [20] and interfacial reactions far from equilibrium. [21] However, X-ray imaging of magnetic materials, still presents a challenge. Acquiring X-rayThe physical properties of magnetic materials frequently depend not only on the microscopic spin and electronic structures, but also on the structures of mesoscopic length scales that emerge, for instance, from domain formations, or chemical and/or electronic phase separations. However, experimental access to such mesoscopic structures is currently limited, especially for antiferromagnets with net zero magnetization. Here, full-field microscopy and resonant magnetic X-ray diffraction are combined to visualize antiferromagnetic (AF) domains of the spin-orbit Mott insulator Sr 2 IrO 4 with area over ≈0.1 mm 2 and ...
