Phenomenological theory of phase transitions in multiferroic MnWO₄: magnetoelectricity and modulated magnetic order.

The phenomenological theory of phase transitions in multiferroic MnWO₄ is suggested. The theoretical model uses the assumption that the magnetic order is driven by the instability in the (1/4;1/2;1/2) point of the Brillouin zone, which is justified by the symmetry of the low-temperature magnetic phase. It is shown that the experimentally observed incommensurate magnetic order is due to the Lifshitz invariants allowed for the corresponding order parameters. Invariants responsible for the magnetoelectric interaction are found and a schematic phase diagram is calculated. The influence of the magnetic field on the phase transition sequence is also analyzed. It is suggested that the description of the phase transitions in MnWO₄ starting from the orthorhombic praphase significantly simplifies the approach and allows us to draw important conclusions.