LANDAU THEORY OF A SYSTEM WITH 2 BILINEARLY COUPLED ORDER PARAMETERS IN EXTERNAL-FIELD - EXACT MEAN-FIELD SOLUTION, CRITICAL PROPERTIES ANDISOTHERMAL SUSCEPTIBILITY

We present simple description of a system with two bilinearly coupled order parameters in external field based on an exact mean-field soluti on of the Landau Hamiltonian. It reproduces the qualitative form of th e ''field-temperature'' phase diagram given by a molecular-field model acid by more sophisticated theories and experiments on metamagnets. T he solution gives the same critical exponents as the molecular-field t heory, but it is not restricted to the magnetic systems only and it is easier to handle, since it formulates the results in explicit analyti cal form. The susceptibility in this model does not diverge at the sec ond order transition line (far from a higher order critical point sepa rating the second and first order transition lines), but jumps down fr om the lower temperature wing to the higher temperature one. The jump amplitude is proportional to the square of the field in small fields a nd diverges in large fields dose to the higher order critical point.