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 , but the low temperature state remained uncertain and was a contentious problem for over 70 years until the full superstructure was determined in 2012 . 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 . Trimerons are examples of orbital molecules, weakly bonded clusters of transition metal ions within an orbitally ordered solid . Recent studies of the orbital molecule orders in synthetic and natural magnetites [5,6], and in other oxides such as the spinel AlV2O4  will be presented. Centre for Science at Extreme Conditions and School of Chemistry, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, UK. email@example.com
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