Spin-ordering in S = 1 anisotropic Heisenberg models with nondiagonal spin exchange

M. Pleimling
Institut für Theoretische Physik, Technische Hochschule Aachen,
D-52056 Aachen, Germany


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 FeBr$_2$ as well as the weakly first-order phase transition observed in the same compound when a tilted field is applied.