  Recent demonstrations of electrical detection and manipulation of
antiferromagnets (AFMs) have opened new opportunities towards robust and
ultrafast spintronics devices. However, it is difficult to establish the
connection between the spin-transport behavior and the microscopic AFM domain
states due to the lack of the real-time AFM domain imaging technique under the
electric field. Here we report a significant Voigt rotation up to 60 mdeg in
thin NiO(001) films at room temperature. Such large Voigt rotation allows us to
directly observe AFM domains in thin-film NiO by utilizing a wide-field optical
microscope. Further complementary XMLD-PEEM measurement confirms that the Voigt
contrast originates from the NiO AFM order. We examine the domain pattern
evolution at a wide range of temperature and with the application of external
magnetic field. Comparing to large-scale-facility techniques such as the X-ray
photoemission electron microscopy, the use with a wide-field, tabletop optical
imaging method enables straightforward access to domain configurations of
single-layer AFMs.
