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

Trevor P. Almeida*, Adrian R. Muxworthy, Takeshi Kasama, Wyn Williams, Christian Damsgaard, Cathrine Frandsen, Timothy J. Pennycook, Rafal E. Dunin-Borkowski

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

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. Key Points: Maghemization of Fe3O4 grains is confirmed through XRD, Mossbauer, and EELS Maghemization of Fe3O4 grains occurs through formation of a core-shell structure Magnetic behavior is considered due to core-shell exchange coupling

Original languageEnglish
Pages (from-to)2969-2979
Number of pages11
JournalGeochemistry, Geophysics, Geosystems
Volume16
Issue number9
Early online date12 Sep 2015
DOIs
Publication statusPublished - 14 Oct 2015

Keywords

  • maghemite
  • magnetite
  • oxidation
  • rock magnetism

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