  Anisotropic antiferromagnetic systems of dimensionality greater than one in
an external field are shown to exhibit a complicated array of ground states
depending on the spin structure of the surface. The simplest structure that
exhibits these effects is the spin ladder with the surface being the ladder
end, which can be either compensated or non-compensated spins. The structure
with the compensated end has a surface spin flop phase, the non-compensated end
has a discommensurational phase, and the transition to these phases can be
either first or second order with a tricritical point.
