  Recent years have witnessed tremendous success in the discovery of
topological states of matter. Particularly, sophisticated theoretical methods
in time-reversal-invariant topological phases have been developed, leading to
the comprehensive search of crystal database and the prediction of thousands of
new topological materials. In contrast, the discovery of magnetic topological
phases that break time reversal is still limited to several exemplary materials
because the coexistence of magnetism and topological electronic band structure
is rare in a single compound. To overcome this challenge, we propose an
alternative approach to realize the quantum anomalous Hall (QAH) effect, a
typical example of magnetic topological phase, via engineering two-dimensional
(2D) magnetic van der Waals heterojunctions. Instead of a single magnetic
topological material, we search for the combinations of two 2D (typically
trivial) magnetic insulator compounds with specific band alignment so that they
can together form a type-III heterojunction with topologically non-trivial band
structure. By combining the data-driven materials search, first principles
calculations, and the symmetry-based analytical models, we identify 8 type-III
heterojunctions consisting of 2D ferromagnetic insulator materials from a
family of 2D monolayer MXY compounds (M = metal atoms, X = S, Se, Te, Y = F,
Cl, Br, I) as a set of candidates for the QAH effect. In particular, we
directly calculate the topological invariant (Chern number) and chiral edge
states in the MnNF/MnNCl heterojunction with ferromagnetic stacking. This work
illustrates how data-driven material science can be combined with
symmetry-based physical principles to guide the search for novel
heterojunction-based quantum materials hosting the QAH effect and other exotic
quantum states in general.

Materials Science