Effect of maghemization on the magnetic properties of nonstoichiometric pseudo-single-domain magnetite particles

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
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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