Landau theory of the magnetic phase diagram of monoclinic multiferroics: Application to MnWO4 and CuO

We present a detailed analysis of the magnetic states of monoclinic multiferroic systems as a function of temperature and magnetic field. The proposed nonlocal Landau free energy derived using symmetry arguments accounts well for the phase diagram of MnWO4 with the field applied along each of the three principal crystallographic directions. A description is proposed for the high field magnetic spin configurations, above 10 T, which have not yet been identified by neutron scattering. The nature of the induced electric polarization in each of the magnetic states is described. Corresponding results for CuO are also presented.