  Using density functional theory and Monte Carlo calculations, we study the
thickness dependence of the magnetic and electronic properties of a van der
Waals interlayer antiferromagnet in the two-dimensional limit. Considering
$\mathrm{MnBi_2Te_4}$ as a model material, we find it to demonstrate a
remarkable set of thickness-dependent magnetic and topological transitions.
While a single septuple layer block of $\mathrm{MnBi_2Te_4}$ is a topologically
trivial ferromagnet, the thicker films made of an odd (even) number of blocks
are uncompensated (compensated) interlayer antiferromagnets, which show wide
bandgap quantum anomalous Hall (zero plateau quantum anomalous Hall) states.
Thus, $\mathrm{MnBi_2Te_4}$ is the first stoichiometric material predicted to
realize the zero plateau quantum anomalous Hall state intrinsically. This state
has been theoretically shown to host the exotic axion insulator phase.
