  Realizing quantum materials in few atomic layer morphologies is a key to both
observing and controlling a wide variety of exotic quantum phenomena. This
includes topological electronic materials, where the tunability and
dimensionality of few layer materials have enabled the detection of $Z_2$,
Chern, and Majorana phases. Here, we report the development of a platform for
thin film correlated, topological states in the magnetic rare-earth
monopnictide ($RX$) system GdBi synthesized by molecular beam epitaxy. This
material is known from bulk single crystal studies to be semimetallic
antiferromagnets with Neel temperature $T_N =$ 28 K and is the magnetic analog
of the non-$f$-electron containing system LaBi proposed to have topological
surface states. Our transport and magnetization studies of thin films grown
epitaxially on BaF$_2$ reveal that semimetallicity is lifted below
approximately 8 crystallographic unit cells while magnetic order is maintained
down to our minimum thickness of 5 crystallographic unit cells.
First-principles calculations show that the non-trivial topology is preserved
down to the monolayer limit, where quantum confinement and the lattice symmetry
give rise to a $C=2$ Chern insulator phase. We further demonstrate the
stabilization of these films against atmospheric degradation using a
combination of air-free buffer and capping procedures. These results together
identify thin film $RX$ materials as potential platforms for engineering
topological electronic bands in correlated magnetic materials.
