Interpretation of magnetoelectric phase states using the praphase concept and exchange symmetry

TL;DR

This paper uses the praphase concept and exchange symmetry to interpret complex magnetoelectric phase diagrams, revealing that these phases can be described by a single irreducible representation related to the praphase or exchange Hamiltonian.

Contribution

It introduces a novel approach employing the praphase concept and exchange symmetry to analyze magnetoelectric phase states, simplifying the understanding of complex phase diagrams.

Findings

Magnetic structures are described by IRs within a single exchange multiplet.

Complex phase diagrams can be interpreted through a single IR of the praphase or exchange Hamiltonian.

Detailed phase diagrams and magnetic structures for MnWO4 and CuO are provided.

Abstract

The majority of magnetoelectric crystals show complex temperature-magnetic field or temperature-pressure phase diagrams with alternating antiferromagnetic incommensurate, magnetoelectric, and commensurate phases. Such phase diagrams occur as a result of successive magnetic instabilities with respect to different order parameters, which usually transform according to different irreducible representations (IR) of the space group of the crystal. Therefore, in order to build a phenomenological theory of phase transitions in such magnetoelectrics one has to employ several order parameters and assume the proximity of various instabilities on the thermodynamic path. In this work we analyze the magnetoelectrics MnWO4, CuO, NaFeSi2O6, NaFeGe2O6, Cu3Nb2O8, alpha-CaCr2O4, and FeTe2O5Br using the praphase concept and the symmetry of the exchange Hamiltonian. We find that in all the considered cases the appearing magnetic structures are described by IR's entering into a single exchange multiplet, whereas in the cases of MnWO4 and CuO by a single IR of the space group of the praphase structure. Therefore, one can interpret the complex phase diagrams of magnetoelectrics as induced by a single IR either of the praphase or of the symmetry group of the exchange Hamiltonian. Detailed temperature-magnetic field phase diagrams of MnWO4 and CuO for certain field directions are obtained and the magnetic structures of the field-induced phases are determined.