Almeida, T. P. , Muxworthy, A. R., Kasama, T., Williams, W., Damsgaard, C., Frandsen, C., Pennycook, T. J. and Dunin-Borkowski, R. E. (2015) Effect of maghemization on the magnetic properties of nonstoichiometric pseudo-single-domain magnetite particles. Geochemistry, Geophysics, Geosystems, 16(9), pp. 2969-2979. (doi: 10.1002/2015GC005858)
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Abstract
The effect of maghemization on the magnetic properties of magnetite (Fe3O4) grains in the pseudo-single-domain (PSD) size range is investigated as a function of annealing temperature. X-ray diffraction and transmission electron microscopy confirm the precursor grains as Fe3O4 ranging from ∼150 to ∼250 nm in diameter, whilst Mössbauer spectrometry suggests the grains are initially near-stoichiometric. The Fe3O4 grains are heated to increasing reaction temperatures of 120–220°C to investigate their oxidation to maghemite (γ-Fe2O3). High-angle annular dark field imaging and localized electron-energy loss spectroscopy reveal slightly oxidized Fe3O4 grains, heated to 140°C, exhibit higher oxygen content at the surface. Off-axis electron holography allows for construction of magnetic induction maps of individual Fe3O4 and γ-Fe2O3 grains, revealing their PSD (vortex) nature, which is supported by magnetic hysteresis measurements, including first-order reversal curve analysis. The coercivity of the grains is shown to increase with reaction temperature up to 180°C, but subsequently decreases after heating above 200°C; this magnetic behavior is attributed to the growth of a γ-Fe2O3 shell with magnetic properties distinct from the Fe3O4 core. It is suggested there is exchange coupling between these separate components that results in a vortex state with reduced vorticity. Once fully oxidized to γ-Fe2O3, the domain states revert back to vortices with slightly reduced coercivity. It is argued that due to a core/shell coupling mechanism during maghemization, the directional magnetic information will still be correct; however, the intensity information will not be retained.
Item Type: | Articles |
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Status: | Published |
Refereed: | Yes |
Glasgow Author(s) Enlighten ID: | Almeida, Dr Trevor |
Authors: | Almeida, T. P., Muxworthy, A. R., Kasama, T., Williams, W., Damsgaard, C., Frandsen, C., Pennycook, T. J., and Dunin-Borkowski, R. E. |
College/School: | College of Science and Engineering > School of Physics and Astronomy |
Journal Name: | Geochemistry, Geophysics, Geosystems |
Publisher: | American Geophysical Union |
ISSN: | 1525-2027 |
ISSN (Online): | 1525-2027 |
Copyright Holders: | Copyright © 2015 The Authors |
First Published: | First published in Geochemistry Geophysics Geosystems 16(9)::2969-2979 |
Publisher Policy: | Reproduced under a Creative Commons License |
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