The Verwey Phase of Magnetite: A Long-Running Mystery in Ferrites

Abstract

Magnetite (Fe 3 O 4 ) is the original magnetic material and the parent of ferrite magnets, with modern applications ranging from spintronics to MRI contrast agents. At ambient temperatures magnetite has a cubic spinel-type crystal structure, but it undergoes a complex structural distortion and becomes electrically insulating below the 125 K Verwey transition. The electronic ground state of the Verwey phase has been unclear for over 70 years as the low temperature structure was unknown, but the full low temperature superstructure was recently determined by high energy microcrystal x-ray diffraction. There are 168 frozen phonon modes in the acentric (and hence multiferroic) low temperature magnetite structure. The ground state was found to be Fe 2+ /Fe 3+ charge ordered and Fe 2+ orbital ordered to a first approximation, but an unexpected localization of electrons in three-Fe 'trimeron' units was discovered.This description is supported by band structure calculations. This brief review will summarise recent progress on understanding the ground state structure of the Verwey phase of magnetite.

Biaxial strain effect induced electronic structure alternation and trimeron recombination in Fe3O4

“…Figure 2 shows the electronic structure of structure (I) and (III). HTP Fe 3 O 4 shows a half metallic characteristic, where the spin-down states near Fermi level comes from the extra minority electron of Fe B t 2 g orbits 2 5 . In Fig.…”

“…As a mixed-valence iron oxide with an inverse spinel lattice, Fe 3 O 4 is formally written as Fe A 3+ [Fe 2+ Fe 3+ ] B O 4 4 . Two Fe B atoms have one spin-down t 2 g electron in 3 d orbits 5 . Rapid hop of the electron between two neighbor Fe B forms the conductive mechanism of Fe 3 O 4 .…”

Magnetoresistance of epitaxial and polycrystalline Fe3O4 films near Verwey transition

“…5 However, magnetite (Fe 3 O 4 ) is an outstanding magnetic material with conductivity which applies in particular to MR. Fe 3 O 4 has attracted much attention because of its unique properties including high Curie temperature (T C % 858 K), 6 full spin polarization at the Fermi level, 7 and a large net magnetic moment of about 4 l B /f.u. 8 In epitaxial Fe 3 O 4 films grown on MgO, Al 2 O 3 , and SrTiO 3 substrates, the mis-stacking of the Fe 3 O 4 lattice on a substrate at the first stage of nucleation could form antiphase boundaries (APBs) which preserve the electronic and magnetic properties of Fe 3 O 4 films away from its bulk one. [9][10][11][12][13][14][15] Across APBs, magnetic moments are coupled antiferromagnetically.…”

Charge localization in the Verwey structure of magnetite

“…At ambient temperatures magnetite has the cubic AB 2 O 4 spinel structure (space group F d3m) with inverse formal charge distribution Fe 3+ (Fe 2.5+ ) 2 O 4 . However, a more complex superstructure is adopted below the Verwey transition at T V ≈ 125 K and the electronic ground state has been investigated for over 75 years since Verwey proposed that this is driven by charge ordering of Fe 2+ and Fe 3+ ions over the octahedral B-type sites in the spinel lattice [1][2][3]. This order was not verified during the early decades of study due to the complexity of the lowtemperature superstructure, which has monoclinic Cc space group symmetry, and to microtwinning of Cc domains.…”