  Topological insulators are new states of matter with a bulk gap and robust
gapless surface states protected by time-reversal symmetry. When time-reversal
symmetry is broken, the surface states are gapped, which induces a topological
response of the system to electromagnetic field--the topological
magnetoelectric effect. In this paper we study the behavior of topological
surface states in heterostructures formed by a topological insulator and a
magnetic insulator. Several magnetic insulators with compatible magnetic
structure and relatively good lattice matching with topological insulators
${\rm Bi_2Se_3}, {\rm Bi_2Se_3}, {\rm Sb_2Te_3}$ are identified, and the best
candidate material is found to be MnSe, an anti-ferromagnetic insulator. We
perform first-principles calculation in ${\rm Bi_2Se_3/MnSe}$ superlattices and
obtain the surface state bandstructure. The magnetic exchange coupling with
MnSe induces a gap of $\sim$54 meV at the surface states. In addition we tune
the distance between Mn ions and TI surface to study the distance dependence of
the exchange coupling.
