Trimeron-phonon coupling in magnetite

TL;DR

This study uses density functional theory to analyze phonon behaviors in magnetite, revealing strong trimeron-phonon coupling that explains experimental observations and extends understanding of its magnetic phase transitions.

Contribution

It demonstrates the significance of trimeron-phonon coupling in magnetite's lattice dynamics across different phases, providing new insights into its complex magnetic properties.

Findings

Good agreement between theory and experiment for monoclinic phase

Discrepancies in cubic phase explained by short-range trimeron effects

Trimeron-phonon coupling influences lattice dynamics beyond the Verwey transition

Abstract

Using density functional theory, we study the lattice dynamical properties of magnetite (Fe$_3$O$_4$) in the high-temperature cubic and low-temperature monoclinic phases. The calculated phonon dispersion curves and phonon density of states are compared with the available experimental data obtained by inelastic neutron, inelastic x-ray, and nuclear inelastic scattering. We find a very good agreement between the theoretical and experimental results for the monoclinic $Cc$ structure revealing the strong coupling between charge-orbital (trimeron) order and specific phonon modes. For the cubic phase, clear discrepancies arise which, remarkably, can be understood assuming that the strong trimeron-phonon coupling can be extended above the Verwey transition, with lattice dynamics influenced by the short-range trimeron order instead of the average cubic structure. Our results establish the validity of trimerons (and trimeron-phonon coupling) in explaining the physics of magnetite much beyond their original formulation.