Phenomenological theory of phase transitions in multiferroicMnWO4: magnetoelectricity and modulated magnetic order

Abstract

The phenomenological theory of phase transitions in multiferroicMnWO4 issuggested. The theoretical model uses the assumption that the magnetic order is driven by the instabilityin the (1/4;1/2;1/2) point of the Brillouin zone, which is justified by the symmetry of the low-temperaturemagnetic phase. It is shown that the experimentally observed incommensurate magneticorder is due to the Lifshitz invariants allowed for the corresponding order parameters.Invariants responsible for the magnetoelectric interaction are found and a schematic phasediagram is calculated. The influence of the magnetic field on the phase transition sequenceis also analyzed. It is suggested that the description of the phase transitions inMnWO4 starting from the orthorhombic praphase significantly simplifies the approach and allows usto draw important conclusions.