  We study the influence of the antiferromagnetic order on the surface states
of topological insulators. We derive an effective Hamiltonian for these states,
taking into account the spatial structure of the antiferromagnetic order. We
obtain a typical (gapless) Dirac Hamiltonian for the surface states when the
surface of the sample is not perturbed. Gapless spectrum is protected by the
combination of time-reversal and half-translation symmetries. However, a shift
in the chemical potential of the surface layer opens a gap in the spectrum away
from the Fermi energy. Such a gap occurs only in systems with finite
antiferromagnetic order. We observe that the system topology remains unchanged
even for large values of the disorder. We calculate the spectrum using the
tight-binding model with different boundary conditions. In this case we get a
gap in the spectrum of the surface states. This discrepancy arises due to the
violation of the combined time-reversal symmetry. We compare our results with
experiments and density functional theory calculations.
