  Topological magnetoelectric effect in a three-dimensional topological
insulator is a novel phenomenon, where an electric field induces a magnetic
field in the same direction, with a universal coefficient of proportionality
quantized in units of $e^2/2h$. Here we propose that the topological
magnetoelectric effect can be realized in the zero-plateau quantum anomalous
Hall state of magnetic topological insulators or ferromagnet-topological
insulator heterostructure. The finite-size effect is also studied numerically,
where the magnetoelectric coefficient is shown to converge to a quantized value
when the thickness of topological insulator film increases. We further propose
a device setup to eliminate the non-topological contributions from the side
surface.
