Co-emergence of magnetic order and structural fluctuations in magnetite

Giuditta Perversi,J Paul Attfield,Elise Pachoud,James Cumby,Simon A J Kimber,Jon P Wright,Jessica M Hudspeth

doi:10.1038/s41467-019-10949-9

Abstract

The nature of the Verwey transition occurring at TV ≈ 125 K in magnetite (Fe3O4) has been an outstanding problem over many decades. A complex low temperature electronic order was recently discovered and associated structural fluctuations persisting above TV are widely reported, but the origin of the underlying correlations and hence of the Verwey transition remains unclear. Here we show that local structural fluctuations in magnetite emerge below the Curie transition at TC ≈ 850 K, through X-ray pair distribution function analysis. Around 80% of the low temperature correlations emerge in proportion to magnetization below TC. This confirms that fluctuations in Fe-Fe bonding arising from magnetic order are the primary electronic instability and hence the origin of the Verwey transition. Such hidden instabilities may be important to other spin-polarised conductors and orbitally degenerate materials.

Highlights

The nature of the Verwey transition occurring at TV ≈ 125 K in magnetite (Fe3O4) has been an outstanding problem over many decades
Analysis of the interatomic pair distribution function (PDF)[11] derived from total X-ray scattering data is a simple method for exploring local structure that is highly sensitive to the displacements of metal atoms associated with the formation of orbital molecules, metal–metal-bonded clusters such as the trimerons observed in magnetite[12]
To fit the PDFs over all temperatures, we have used this monoclinic supercell with lattice parameters adjusted to a cubic metric and with each of the 168 atomic coordinates given by p = pu + fV(pd − pu) where pu is the coordinate from an undistorted hightemperature cubic crystal structure refinement and pd is the coordinate in the distorted 90 K structure reported previously3. fV is a Verwey shift parameter that describes the set of constrained structural displacements, such that fV = 0 corresponds to the cubic spinel structure without any local distortions and fV = 1 describes the full magnitude of distortions in the 90 K magnetite superstructure[3]

Summary

Introduction

The nature of the Verwey transition occurring at TV ≈ 125 K in magnetite (Fe3O4) has been an outstanding problem over many decades. A complex low temperature electronic order was recently discovered and associated structural fluctuations persisting above TV are widely reported, but the origin of the underlying correlations and of the Verwey transition remains unclear. We show that local structural fluctuations in magnetite emerge below the Curie transition at TC ≈ 850 K, through X-ray pair distribution function analysis. Around 80% of the low temperature correlations emerge in proportion to magnetization below TC This confirms that fluctuations in Fe-Fe bonding arising from magnetic order are the primary electronic instability and the origin of the Verwey transition. Such hidden instabilities may be important to other spin-polarised conductors and orbitally degenerate materials. The thermal variation of local structure in magnetite has been explored here over a wide temperature range encompassing both the Verwey and Curie transitions through synchrotron X-ray PDF analysis

Methods

Results

Conclusion