  Heterostructures between topological insulators (TI) and magnetic insulators
represent a pathway to realize the quantum anomalous Hall effect (QAHE). Using
density functional theory based systematic screening and investigation of
thermodynamic, magnetic and topological properties of heterostructures, we
demonstrate that forming a type-I heterostructure between a wide gap
antiferromagnetic insulator Cr$_2$O$_3$ and a TI-film, such as Sb$_2$Te$_3$,
can lead to pinning of the Fermi-level at the center of the gap, even when
magnetically doped. Cr-doping in the heterostructure increases the gap to
$\sim$ 64.5 meV, with a large Zeeman energy from the interfacial Cr dopants,
thus overcoming potential metallicity due to band bending effects. By fitting
the band-structure around the Fermi-level to a 4-band k.p model Hamiltonian, we
show that Cr doped Sb$_2$Te$_3$/Cr$_2$O$_3$ is a Chern insulator with a Chern
number C = -1. Transport calculations further show chiral edge-modes localized
at the top/bottom of the TI-film to be the dominant current carriers in the
material. Our predictions of a large interfacial magnetism due to Cr-dopants,
that coupled antiferromagnetically to the AFM substrate is confirmed by our
polarised neutron reflectometry measurements on MBE grown Cr doped
Sb$_2$Te$_3$/Cr$_2$O$_3$ heterostructures, and is consistent with a positive
exchange bias measured in such systems recently. Consequently, Cr doped
Sb$_2$Te$_3$/Cr$_2$O$_3$ heterostructure represents a promising platform for
the development of functional topological magnetic devices, with high
tunability.
