Nonlocal Landau theory of the magnetic phase diagram of highly frustrated magnetoelectricCuFeO2

Abstract

A nonlocal Landau-type free-energy functional of the spin density is developed to model the large variety of magnetic states, which occur in the magnetic-field\char21{}temperature phase diagram of magnetoelectric ${\text{CuFeO}}_{2}$. Competition among long-range quadratic exchange, biquadratic antisymmetric exchange, and trigonal anisotropy terms, consistent with the high-temperature rhombohedral $R\overline{3}m$ crystal symmetry, are shown to all play important roles in stabilizing the unusual combination of commensurate and incommensurate spin structures in this highly frustrated triangular antiferromagnet. It is argued that strong magnetoelastic coupling is largely responsible for the nonlocal nature of the free energy. A key feature of the analysis is that an electric polarization is induced by a canting of the noncollinear incommensurate spin structure. Application of the model to ordered spin states in the triangular antiferromagnets ${\text{MnBr}}_{2}$ and ${\text{NaFeO}}_{2}$ is also discussed.