Aspects of the Verwey transition in magnetite

Seo, Hitoshi; Ogata, Masao; Fukuyama, Hidetoshi

doi:10.1103/physrevb.65.085107

Cited by 63 publications

(42 citation statements)

References 48 publications

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“…13 One could argue that similar mechanisms are at work here. However, concerning the latter, there is no substantiated theoretical or experimental hint for a CDW bulk phase in magnetite-e.g., because of hidden one dimensionality 23 -which could induce a surface CDW due to a kind of proximity effect, nor is there any reason to expect a genuine surface Peierls phase due to the fact that the specific surface electronic structure would favor Fermi surface nesting conditions in contrast to the bulk. Actually, there exist only very few and atomically "clean" metal or metalsurface systems, which were discussed under the notion of surface Peierls phases.…”

Section: Superstructurementioning

confidence: 99%

Thermodynamic stability and atomic and electronic structure of reducedFe3O4(111) single-crystal surfaces

et al. 2007

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Citation for published version (APA):Paul, M., Sing, M., Claessen, R., Schrupp, D., & Brabers, V. A. M. (2007). Thermodynamic stability and atomic and electronic structure of reduced Fe3 O4 (111) Please check the document version of this publication:• A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal.If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User Agreement: www.tue.nl/taverne Take down policyIf you believe that this document breaches copyright please contact us at:openaccess@tue.nl providing details and we will investigate your claim. Magnetite ͑111͒ single-crystal surfaces prepared in situ under different reducing conditions and-as a result-with varying stoichiometries have been studied by scanning tunneling microscopy, low-energy electron diffraction, and x-ray photoemission spectroscopy. The coexistence of several surface structures has been detected, indicating only small differences in their relative stabilities. In particular, an unusual previously unreported superstructure has been found for a strongly reduced surface. Its microscopic origin is discussed against the background of recent results from scanning tunneling microscopy of the oxidized magnetite ͑111͒ surface and from ab initio thermodynamics. Partly at variance with and partly complementary to these results, we regard as driving force elastic strain due to the lateral mismatch between Fe 3 O 4 substrate and Fe 1−x O-like overlayer.

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Section: Superstructurementioning

confidence: 99%

Thermodynamic stability and atomic and electronic structure of reducedFe3O4(111) single-crystal surfaces

et al. 2007

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“…55 Indeed, the polaronic character of charge carriers has been observed in the optical conductivity 48,56 and in photoemision studies. 34 The cooperative mechanism of the VT was next studied also in the Peierls-Hubbard model, 57 where strong on-site interaction induces the OO and lattice distortion. Since the active electrons on Fe +2 ions occupy partly filled degenerate t 2g states, the EP coupling could be further enhanced by the Jahn-Teller effect.…”

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Origin of the Verwey transition in magnetite: Group theory, electronic structure, and lattice dynamics study

2007

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The Verwey phase transition in magnetite has been analyzed using the group theory methods. It is found that two order parameters with the symmetries X3 and ∆5 induce the structural transformation from the high-temperature cubic to the low-temperature monoclinic phase. The coupling between the order parameters is described by the Landau free energy functional. The electronic and crystal structure for the cubic and monoclinic phases were optimized using the ab initio density functional method. The electronic structure calculations were performed within the generalized gradient approximation including the on-site interactions between 3d electrons at iron ions -the Coulomb element U and Hund's exchange J. Only when these local interactions are taken into account, the phonon dispersion curves, obtained by the direct method for the cubic phase, reproduce the experimental data. It is shown that the interplay of local electron interations and the coupling to the lattice drives the phonon order parameters and is responsible for the opening of the gap at the Fermi energy. Thus, it is found that the metal-insulator transition in magnetite is promoted by local electron interactions, which significantly amplify the electron-phonon interaction and stabilize weak charge order coexisting with orbital order of the occupied t2g states at Fe ions. This provides a scenario to understand the fundamental problem of the origin of the Verwey transition in magnetite.

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“…We emphasize that the present mechanism is novel and does not benefit from the Peierls-like distortions. Such a cooperative mechanism including strong electron correlations and the EP coupling was studied before [30], but the dimer-bond formation seems not to be supported experimentally [3].…”

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Mechanism of the Verwey Transition in Magnetite

2006

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By combining ab initio results for the electronic structure and phonon spectrum with the group theory, we establish the origin of the Verwey transition in Fe3O4. Two primary order parameters with X3 and ∆5 symmetries are identified. They induce the phase transformation from the hightemperature cubic to the low-temperature monoclinic structure. The on-site Coulomb interaction U between 3d electrons at Fe ions plays a crucial role in this transition -it amplifies the coupling of phonons to conduction electrons and thus opens a gap at the Fermi energy. Published in Phys. Rev. Lett. 97, 156402 (2006). PACS numbers: 71.30.+h, 64.70.Kb, 75.50.Gg The discovery of the remarkable discontinuous drop of the electrical conductivity by two orders of magnitude in magnetite (Fe 3 O 4 ) below the Verwey transition (VT) at T V = 122 K [1] triggered intensive studies of its microscopic origin which have been continued for more than six decades. Verwey suggested explaining it by a metalinsulator transition due to the reduction of electron mobility caused by ordering of Fe 3+ and Fe 2+ ions below T V . In spite of great experimental effort, however, the existence of ionic-like charge ordering at T < T V could not be confirmed, and the origin of the VT in magnetite remains still puzzling [2].Magnetite is a ferrimagnetic spinel with anomalously high critical temperature T c ≃ 860 K. Hence, it is viewed as an ideal candidate for room temperature spintronic applications. It crystalizes in the inverse spinel cubic structure, Fd3m, with two types of Fe sites: the tetrahedral A sites and the octahedral B ones, see Ref. There are also other arguments against simple ionic mechanism of the VT. Firstly, the replacement of oxygen O 16 by O 18 isotope increases T V by a few degrees [9], indicating that the transition cannot be purely electronic.Secondly, signals in diffuse neutron scattering observed above T V at k ∆ = (0, 0, 1 2 ), halfway between Γ and X points with k X = (0, 0, 1), and equivalent points of the reciprocal lattice [10] (in units of 2π a , where a is the cubic lattice constant), change into Bragg peaks below T V . The intensity of critical scattering was succesfully calculated on the basis of the transverse acoustic (TA) phonon mode with the ∆ 5 symmetry. Further neutron studies discovered diffuse scattering with large intensities close to Γ and X points, and the dominant component of X 3 symmetry [11]. Other observations from Raman [12], X-ray [13] and nuclear inelastic scattering measurements [14] suggest that phonons play an essential role in the VT. But phonons alone cannot explain the metal-insulator nature of the transition either. Actually, if they alone were responsible, a phonon soft mode would have been found for the cubic phase, while the present ab initio calculations do not imply such a soft mode behavior.The purpose of this Letter is to resolve the above controversies and to demonstrate a cooperative scenario of the VT. Following the pioneering work of Ihle and Lorenz [15], who considered weak intersite Coulom...

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Aspects of the Verwey transition in magnetite

Cited by 63 publications

References 48 publications

Thermodynamic stability and atomic and electronic structure of reducedFe3O4(111) single-crystal surfaces

Thermodynamic stability and atomic and electronic structure of reducedFe3O4(111) single-crystal surfaces

Origin of the Verwey transition in magnetite: Group theory, electronic structure, and lattice dynamics study

Mechanism of the Verwey Transition in Magnetite

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