Distribution of antiferromagnetic spin and twin domains in NiO

Abstract

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 $\ensuremath{\lesssim}1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$. NiO samples annealed in oxygen possess a distribution of $T$ domains of $\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ made up by a random distribution of the three corresponding $S$ domains of $\ensuremath{\lesssim}1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$. Only next to a $T$ domain wall are ``large'' $S$ domains of $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ formed because of a tendency of the ${\mathrm{Ni}}^{2+}$ 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.