Zener double exchange from local valence fluctuations in magnetite

TL;DR

This paper investigates how valence fluctuations in magnetite influence magnetic excitations, revealing that double exchange interactions emerge above the Verwey transition and selectively affect certain spin wave modes.

Contribution

It provides experimental evidence linking local valence fluctuations to double exchange interactions in magnetite, highlighting their role in the metal-insulator transition.

Findings

Optical spin waves are shifted upwards above T_V due to double exchange.

Double exchange affects only specific spin wave modes with Δ_5 symmetry.

Valence fluctuations are slow and constrained by electron correlations above T_V.

Abstract

Magnetite (Fe$_{3}$O$_{4}$) is a mixed valent system where electronic conductivity occurs on the B-site (octahedral) iron sublattice of the spinel structure. Below $T_{V}=122$ K, a metal-insulator transition occurs which is argued to arise from the charge ordering of 2+ and 3+ iron valences on the B-sites (Verwey transition). Inelastic neutron scattering measurements show that optical spin waves propagating on the B-site sublattice ($\sim$80 meV) are shifted upwards in energy above $T_{V}$ due to the occurrence of B-B ferromagnetic double exchange in the mixed valent metallic phase. The double exchange interaction affects only spin waves of $\Delta_{5}$ symmetry, not all modes, indicating that valence fluctuations are slow and the double exchange is constrained by electron correlations above $T_{V}$.