Distribution of antiferromagnetic spin and twin domains in NiO

The distribution of antiferromagnetic spin $(S)$ and twin $(T)$ domains in NiO is investigated by optical second-harmonic generation (SHG). The 12 orientational domains as well as typical distributions of domains can be distinguished by SHG. Untreated NiO samples possess a random distribution of all S and T domains with a lateral size on the order of 10.3em0exμm. NiO samples annealed in oxygen possess a distribution of T domains of ~1000.3em0exμm made up by a random distribution of the three corresponding S domains of 10.3em0exμm. Only next to a T domain wall are ``large'' S domains of ~100.3em0exμm formed because of a tendency of the Ni²⁺ spins to orient along the stacking direction of the ferromagnetic planes in the adjacent T domain. It is concluded that SHG is not only a powerful addition to alternative techniques for probing domain structures in NiO but, in fact, indispensable for studying contemporary aspects of antiferromagnetism.