Charge and Spin Ordering in the Mixed Valence Compound LuFe2O4

TL;DR

This paper explains the complex charge and spin ordering phenomena in LuFe2O4 using Landau theory, symmetry, and first-principles calculations, highlighting incommensurate order and the role of electron-phonon interactions.

Contribution

It introduces a comprehensive theoretical framework combining Landau theory, symmetry, and first-principles calculations to explain charge and spin ordering in LuFe2O4, including incommensurate phases.

Findings

Both charge and spin orderings are incommensurate.

Electron-phonon interactions stabilize certain ordered phases.

First-principles phonon energies support the theoretical model.

Abstract

Landau theory and symmetry considerations lead us to propose an explanation for several seemingly paradoxical behaviors of charge ordering (CO) and spin ordering (SO) in the mixed valence compound LuFe2O4. Both SO and CO are highly frustrated. We analyze a lattice gas model of CO within mean field theory and determine the magnitude of several of the phenomenological interactions. We show that the assumption of a continuous phase transition at which CO or SO develops implies that both CO and SO are incommensurate. To explain how ferroelectric fluctuations in the charge disordered phase can be consistent with an antiferroelectric ordered phase, we invoke an electron-phonon interaction in which a low energy (20meV) zone-center transverse phonon plays a key role. The energies of all the zone-center phonons are calculated from first principles. We give a Landau analysis which explains SO and we discuss a model of interactions which stabilizes the SO state, if it is assumed commensurate. However, we suggest a high resolution experimental determination to see whether this phase is really commensurate, as believed up to now. The applicability of representation analysis is discussed. A tentative explanation for the sensitivity of the CO state to an applied magnetic field in field-cooled experiments is given.