A magnetic lattice gas of composite particles in a transverse field

A model of a lattice gas of molecules consisting of two or three atoms in a transverse field has been considered. Using the molecular field approximation for long-ranged inter-molecular forces the dependence on temperature and transverse field of the parallel and perpendicular magnetisations, as well as of the susceptibilities has been determined. Introduction of short-ranged, intra-molecular forces, leads, for a certain range of parameters, to an increase with the temperature of the perpendicular magnetisation. A phase diagram for the model has been also constructed.