Field and surface effects on the ground state of antiferromagnetic systems

The field dependence of the ground state of semi-infinite uniaxial antiferromagnets and films is investigated in terms of a two-dimensional area-preserving map, where the surfaces are introduced as appropriate boundary conditions. For the film, the ground state is calculated for any value of the number of planes in a very rapid and accurate way. For the semi-infinite system, we show that the so-called surface spin-flop state does not exist; by energetic arguments, we find that the instability of the antiferromagnetic configuration leads to an interchange of the sublattices. A nonhomogeneous ground state is instead found in the bulk spin-flop phase.