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School of Physics and Astronomy

Orbital molecules in magnetite and beyond

13 February 2018

Time: 1:45 - 3:45pm
Speaker: Prof J. Paul Attfield
Venue: G. O. Jones Building, Room 610

Magnetite (Fe3O4) is the original magnetic material and remains fundamental to understanding of magnetism. On cooling below 125 K, magnetite undergoes a complex structural distortion accompanied by changes in the thermal expansion coefficient, and becomes electrically insulating. Verwey proposed in 1939 that this transition is driven by a charge ordering of Fe2+ and Fe3+ ions [1], but the low temperature state remained uncertain and was a contentious problem for over 70 years until the full superstructure was determined in 2012 [2]. Verwey’s charge order hypothesis was found to be approximately correct, with a pronounced orbital ordering of Fe2+ states, but an unexpected localization of electrons in linear, three-Fe ‘trimeron’ units was discovered and is supported by band structure calculations [3]. Trimerons are examples of orbital molecules, weakly bonded clusters of transition metal ions within an orbitally ordered solid [4]. Recent studies of the orbital molecule orders in synthetic and natural magnetites [5,6], and in other oxides such as the spinel AlV2O4 [7] will be presented. Centre for Science at Extreme Conditions and School of Chemistry, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, UK.

[1] Verwey, E.J.W. (1939). Nature 144, 327-328.

[2] Senn, M.S., Wright, J.P. & Attfield J.P. (2012), Nature 481, 173-176.

[3] Senn, M.S., Loa, I., Wright, J.P. & Attfield J.P. (2012). Phys. Rev. B 85, 125119.

[4] J. P. Attfield (2015). Applied Physics Letters Materials 3, 041510.

[5] Senn, M.S.,Wright, J.P., Cumby, J. & Attfield J.P. (2015). Phys. Rev. B 92, 024104.

[6] G. Perversi , J. Cumby , E. Pachoud , J. P. Wright, J. P. Attfield (2016). Chem. Comm. 52, 4864-4867.

[7] Browne, A. J.; Kimber, S. A. J.; Attfield, J. P. (2017). Phys. Rev. Mater., 1, 052003(R).