  The properties of S = 1 anisotropic Heisenberg models with nondiagonal
exchange between axial and planar spin components are investigated using Monte
Carlo techniques. The quantum nature is taken into account in a semi-classical
approximation. The ordering of the spins when applying an external field with
axial and planar components is discussed. It is argued that the quantum nature
of the spins and the nondiagonal exchange may explain the peculiar shape of the
magnetic specific heat of FeBr2 as well as the weakly first-order phase
transition observed in the same compound when a tilted field is applied.
