---
abstract: |
    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.\
address: 'Institut für Theoretische Physik B, Technische Hochschule, D–52056 Aachen, Germany'
author:
- 'M. Pleimling'
title: 'Spin-ordering in S = 1 anisotropic Heisenberg models with nondiagonal spin exchange'
---

Introduction
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Metamagnets of Ising type display a field–induced phase transition from an antiferromagnetic phase to a (saturated) paramagnetic phase, the transition being of first order at low temperatures and of second order at high temperatures.[@kin75; @str77] The first and second order transition lines meet at a tricritical point. The layered antiferromagnet FeCl$_2$ is generally considered to be the textbook example of this kind of antiferromagnets with a strong uniaxial anisotropy.[@bir72]

