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

Trevor P. Almeida, Adrian R. Muxworthy, Takeshi Kasama, W. Williams, Christian Danvad Damsgaard, Cathrine Frandsen, T.J. Pennycook, R.E. Dunin-Borkowski

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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 1808°C, but subsequently decreases after heating above 200°; 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.
Original languageEnglish
JournalGeochemistry, Geophysics, Geosystems
Volume16
Issue number9
Pages (from-to)2969-2979
Number of pages11
ISSN1525-2027
DOIs
Publication statusPublished - 2015

Bibliographical note

© 2015. The Authors. This is an open access article under the terms of the Creative Commons Attribution License.

Cite this

Almeida, Trevor P. ; Muxworthy, Adrian R. ; Kasama, Takeshi ; Williams, W. ; Damsgaard, Christian Danvad ; Frandsen, Cathrine ; Pennycook, T.J. ; Dunin-Borkowski, R.E. / Effect of maghemization on the magnetic properties of nonstoichiometric pseudo-single-domain magnetite particles. In: Geochemistry, Geophysics, Geosystems. 2015 ; Vol. 16, No. 9. pp. 2969-2979.
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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 M{\"o}ssbauer 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 1808°C, but subsequently decreases after heating above 200°; 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.",
author = "Almeida, {Trevor P.} and Muxworthy, {Adrian R.} and Takeshi Kasama and W. Williams and Damsgaard, {Christian Danvad} and Cathrine Frandsen and T.J. Pennycook and R.E. Dunin-Borkowski",
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Effect of maghemization on the magnetic properties of nonstoichiometric pseudo-single-domain magnetite particles. / Almeida, Trevor P.; Muxworthy, Adrian R.; Kasama, Takeshi; Williams, W.; Damsgaard, Christian Danvad; Frandsen, Cathrine; Pennycook, T.J.; Dunin-Borkowski, R.E.

In: Geochemistry, Geophysics, Geosystems, Vol. 16, No. 9, 2015, p. 2969-2979.

Research output: Contribution to journalJournal articleResearchpeer-review

TY - JOUR

T1 - Effect of maghemization on the magnetic properties of nonstoichiometric pseudo-single-domain magnetite particles

AU - Almeida, Trevor P.

AU - Muxworthy, Adrian R.

AU - Kasama, Takeshi

AU - Williams, W.

AU - Damsgaard, Christian Danvad

AU - Frandsen, Cathrine

AU - Pennycook, T.J.

AU - Dunin-Borkowski, R.E.

N1 - © 2015. The Authors. This is an open access article under the terms of the Creative Commons Attribution License.

PY - 2015

Y1 - 2015

N2 - 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 1808°C, but subsequently decreases after heating above 200°; 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.

AB - 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 1808°C, but subsequently decreases after heating above 200°; 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.

U2 - 10.1002/2015gc005858

DO - 10.1002/2015gc005858

M3 - Journal article

VL - 16

SP - 2969

EP - 2979

JO - Geochemistry, Geophysics, Geosystems

JF - Geochemistry, Geophysics, Geosystems

SN - 1525-2027

IS - 9

ER -