  Motivated by the goal of realizing topological phases in thin films and
heterostructures of correlated oxides, we propose here a quantum anomalous Hall
insulator (QAHI) in ultrathin films of double perovskites based on mixed 3d-5d
or 3d-4d transition metal ions, grown along the [111] direction. Considering
the specific case of ultrathin Ba2FeReO6, we present a theoretical analysis of
an effective Hamiltonian derived from first-principles. We establish that a
strong spin-orbit coupling at Re site, t2g symmetry of the low-energy d-bands,
polarity of its [111] orientation of perovskite structure, and mixed 3d-5d
chemistry results in room temperature magnetism with a robust QAHI state of
Chern number C=1 and a large band-gap. We uncover and highlight a
non-relativistic orbital-Rashba-type effect in addition to the spin-orbit
coupling, that governs this QAHI state. Our prediction of a large topological
band-gap of ~100 meV in electronic structure, and a magnetic transition
temperature Tc~300K estimated by Monte Carlo simulations, is expected to
stimulate experimental efforts at synthesis of such films and enable possible
practical applications of its dissipationless edge currents.
