Recent demonstrations of electrical detection and manipulation of antiferromagnets (AFMs) have opened new opportunities toward robust and ultrafast spintronics devices. However, it is difficult to establish the connection between the spin-transport behavior and the microscopic AFM domain states in thin films due to the lack of a real-time imaging technique under the electric field. Here we report a large magneto-optical birefringence effect with polarization rotation up to 60 millidegrees in thin NiO(001) films at room temperature. Such large optical polarization rotation allows us to directly observe AFM domains in thin-film NiO by utilizing a wide-field optical microscope. Complementary x-ray magnetic linear dichroism-photoemission electron microscopy measurement further confirms that the optical contrast is related to the NiO AFM domain. We examine the domain pattern evolution at a wide range of temperatures and with the application of external magnetic field. Comparing to large-scale-facility techniques such as x-ray photoemission electron microscopy, using a wide-field, tabletop optical imaging method in reflection geometry enables straightforward access to domain configurations of single-layer AFMs
