Praphase concept for the phenomenological description of magnetoelectrics

Complex cascades of phase transitions with the alternation of antiferromagnetic, incommensurate, and magnetoelectric phases are observed in most crystals having magnetoelectric properties. The entire variety of observed phase transitions in numerous recent works is described by introducing several order parameters which are sequentially “condensed” with a decrease in temperature. The possibility of introducing the praphase concept into consideration to reduce the number of order parameters and interpret rich phase diagrams due to the smaller number of instabilities is demonstrated by the examples of MnWO4 and CuO magnetoelectrics. This approach is also used in the phenomenological description of the phase transitions in the BiFeO3 ferroelectromagnetic with the cubic phase of the perovskite structure as a starting point.