Nonlocal Landau theory of the magnetic phase diagram of highly frustrated magnetoelectric CuFeO$_2$

TL;DR

This paper develops a nonlocal Landau theory to explain the complex magnetic phase diagram of CuFeO$_2$, emphasizing the roles of various exchange interactions and magnetoelastic coupling in stabilizing diverse spin structures.

Contribution

It introduces a nonlocal Landau free energy functional incorporating multiple exchange interactions and anisotropies, explaining the stabilization of complex magnetic states in CuFeO$_2$.

Findings

Magnetoelastic coupling causes nonlocal free energy behavior.

Competition among exchange interactions stabilizes diverse spin structures.

Electric polarization is induced by non-collinear spin canting.

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-temperature phase diagram of magnetoelectric CuFeO$_2$. Competition among long-range quadratic exchange, biquadratic anti-symmetric exchange, and trigonal anisotropy terms, consistent with the high-temperature rhombohedral R$\bar{3}$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 non-collinear incommensurate spin structure. Application of the model to ordered spin states in the triangular antiferromagnets MnBr$_2$ and NaFeO$_2$ is also discussed.