Electronic orders in the Verwey structure of magnetite

Abstract

Electronic structure calculations of the Verwey ground state of magnetite, Fe 3 O 4 , using density functional theory with treatment of on-site Coulomb interactions (DFT + U scheme), are reported. These calculations use the recently published experimental crystal structure coordinates for magnetite in the monoclinic space group Cc. The computed density distribution for minority spin electron states close to the Fermi level demonstrates that charge order and Fe 2+ -orbital order are present at the B-type lattice sites to a first approximation. However, Fe 2+ /Fe 3+ charge differences are diminished through weak bonding interactions of the Fe 2+ states to specific pairs of neighboring iron sites that create linear, three-B-atom trimeron units that may be regarded as orbital molecules. Trimerons are ordered evenly along most Fe atom chains in the Verwey structure, but more complex interactions are observed within one chain.

The verwey phase of magnetite — a long-running mystery in magnetism

“…states, but are instead spread over three sites resulting in highly structured three-site polarons termed 'trimerons'. Electronic structure calculations for the experimental Cc structural model support the first-approximation charge and orbital order model, and also the fuller trimeron description [15]. The apparent dominance of trimeron order within the Verwey structure of magnetite was not proposed in previous theoretical or experimental studies.…”

Anisotropic magnetoresistance across Verwey transition in charge ordered Fe3O4 epitaxial films

“…Although the Verwey transition has been observed for nearly 80 years, its mechanism is still debated. Recently, Senn et al [11,12] have found that the bond length of Fe B is localized in the three-Fe-site unit called a trimeron, which is different from the previous dimerization model [13]. The novel microstructure may provide a way to understand the Verwey transition.…”

“…Remarkably, AMR turns from twofold to fourfold symmetry in Fe 3 O 4 (100) film below T V . We propose that trimerons [11,12] play an important role in fourfold AMR below T V .…”

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

“…The complex charge ordering pattern was found to have been correctly predicted by optimising atomic coordinates within the experimental Cc supercell in two recent band structure calculations that used density functional theory with treatment of on-site Coulomb interactions (DFT+U method) [ 31 , 32 ], and the observed orbital order was also predicted in [32]. [33]. Atomic displacements due to trimeron order result in a large predicted ferroelectric polarisation of ~0.4 Cm -2 so multiferroic behaviour with strong magnetoelectric coupling may be observable in the Verwey phase of magnetite.…”

“…Fig. 4 The charge ordered Verwey structure recently determined for magnetite [29], showing first approximation Fe The trimerons observed in the low temperature magnetite structure may be described as 'orbital molecules' -local molecular entities of weakly-bonded orbitally-ordered ions [33]. Previously reported species that may be classified as orbital molecules are non-magnetic, usually two-electron dimers, e.g.…”