  Realizing axion insulator state with a uniform magnetization considerably
facilitates experimental explorations of the intriguing topological
magnetoelectric effect, a hallmark of three-dimensional (3D) topological
insulators (TIs). Through density functional theory calculations and four-band
model studies, we find that magnetic ions Cr3+ in monolayer CrI3 and Mn2+ in
septuple-layer MnBi2Se4 have opposite exchange couplings to the topological
surface states of 3D TI Bi2Se3. As an exciting result of such opposite exchange
couplings, axion insulator state is realized by a uniform magnetization in
CrI3/Bi2Se3/MnBi2Se4 heterostructure. Our work opens up opportunities for
exploring topological magnetoelectric effect realized by the uniform
magnetization induced axion insulator state in heterostructures of 3D TIs and
two-dimensional van der Waals ferromagnetic insulators.
