  In this work, we present electronic and magnetic properties of CaMnO3 (CMO)
as obtained from ab initio calculations. We identify the preferable magnetic
order by means of density functional theory plus Hubbard U calculations and
extract the effective exchange parameters (Jij's) using the magnetic force
theorem. We find that the effects of geometrical relaxation at the surface as
well as the change of crystal field are very strong and are able to influence
the lower energy magnetic configuration. In particular, our analysis reveals
that the exchange interaction between the Mn atoms belonging to the surface and
the subsurface layers is very sensitive to the structural changes. An earlier
study [A. Filippetti and W.E. Pickett, Phys. Rev. Lett. 83, 4184 (1999)]
suggested that this coupling is ferromagnetic and gives rise to the spin flip
process on the surface of CMO. In our work we confirm their finding for an
unrelaxed geometry, but once the structural relaxations are taken into account,
this exchange coupling changes its sign. Thus, we suggest that the surface of
CMO should have the same G-type antiferromagnetic order as in the bulk.
Finally, we show that the suggested SF can be induced in the system by
introducing an excess of electrons.
