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Visualized effect of oxidation on magnetic recording fidelity in pseudo-single-domain magnetite particles.

Almeida TP, Kasama T, Muxworthy AR, Williams W, Nagy L, Hansen TW, Brown PD, Dunin-Borkowski RE - Nat Commun (2014)

Bottom Line: Magnetite (Fe3O4) is an important magnetic mineral to Earth scientists, as it carries the dominant magnetic signature in rocks, and the understanding of its magnetic recording fidelity provides a critical tool in the field of palaeomagnetism.However, reliable interpretation of the recording fidelity of Fe3O4 particles is greatly diminished over time by progressive oxidation to less magnetic iron oxides, such as maghemite (γ-Fe2O3), with consequent alteration of remanent magnetization potentially having important geological significance.Here we use the complementary techniques of environmental transmission electron microscopy and off-axis electron holography to induce and visualize the effects of oxidation on the magnetization of individual nanoscale Fe3O4 particles as they transform towards γ-Fe2O3.

View Article: PubMed Central - PubMed

Affiliation: Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK.

ABSTRACT
Magnetite (Fe3O4) is an important magnetic mineral to Earth scientists, as it carries the dominant magnetic signature in rocks, and the understanding of its magnetic recording fidelity provides a critical tool in the field of palaeomagnetism. However, reliable interpretation of the recording fidelity of Fe3O4 particles is greatly diminished over time by progressive oxidation to less magnetic iron oxides, such as maghemite (γ-Fe2O3), with consequent alteration of remanent magnetization potentially having important geological significance. Here we use the complementary techniques of environmental transmission electron microscopy and off-axis electron holography to induce and visualize the effects of oxidation on the magnetization of individual nanoscale Fe3O4 particles as they transform towards γ-Fe2O3. Magnetic induction maps demonstrate a change in both strength and direction of remanent magnetization within Fe3O4 particles in the size range dominant in rocks, confirming that oxidation can modify the original stored magnetic information.

No MeSH data available.


EEL spectra acquired from reference iron oxide samples.EELS analysis of the Fe 2p L2,3 edge experimentally acquired from pure samples of Fe3O4 (blue), γ-Fe2O3 (red) and α-Fe2O3 (black).
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f2: EEL spectra acquired from reference iron oxide samples.EELS analysis of the Fe 2p L2,3 edge experimentally acquired from pure samples of Fe3O4 (blue), γ-Fe2O3 (red) and α-Fe2O3 (black).

Mentions: The bright-field TEM image of Fig. 1b shows the same Fe3O4 grain after exposure to 9 mbar O2 atmosphere at 700 °C for 8 h within the ETEM. Degradation of the surface of the grain is apparent, while the associated SAED pattern (Fig. 1b, inset) does not present any evidence for the formation of additional crystalline phases, with brightening of some planar reflections (for example, the −2 8 6 reflection) being attributable to slight tilting of the grain during annealing. However, the development of fine features in the associated EEL spectrum of the heated grain, taking the form of a small pre-peak in the L3 edge and post-peak in the L2 edge (Fig. 1c, red arrows), is indicative of a change in the Fe oxidation state towards γ-Fe2O3 or α-Fe2O3 (refs 15, 16, 17), as illustrated by the reference iron oxide EEL spectra displayed in Fig. 2. It is recognized that progressive oxidation induces the development of these small peaks, with complete oxidation to γ-Fe2O3 being associated with an ~1.3 eV splitting in the L3 edge15. Similarly, the Fe 2p edge in an α-Fe2O3 EEL spectrum is associated with a strong pre-peak located ~1.6 eV in front of the L3 edge1617. Various iron hydroxides can also present similar pre-peaks in their Fe L3 edges; however, O2 is the only gas introduced into the system between acquisitions of EEL spectra, and hence the evolution of pre-peaks in this case is attributed solely to the effects of oxidation. The spacings between the central magnetic contours in the corresponding magnetic induction map (Fig. 1e), again flowing in a counterclockwise direction, were found to widen, most markedly towards the particle edge. Figure 1f,g presents magnetic contributions to the phase shifts used to construct Fig. 1d,e, respectively, and the reduction in amplitudes of the line profiles across their centres (dashed lines) is a strong indicator for loss of overall magnetic remanence in the Fe3O4 particle, as a consequence of oxidation (Fig. 1h, arrowed), again consistent with the progressive conversion of Fe3O4 towards γ-Fe2O3.


Visualized effect of oxidation on magnetic recording fidelity in pseudo-single-domain magnetite particles.

Almeida TP, Kasama T, Muxworthy AR, Williams W, Nagy L, Hansen TW, Brown PD, Dunin-Borkowski RE - Nat Commun (2014)

EEL spectra acquired from reference iron oxide samples.EELS analysis of the Fe 2p L2,3 edge experimentally acquired from pure samples of Fe3O4 (blue), γ-Fe2O3 (red) and α-Fe2O3 (black).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4214405&req=5

f2: EEL spectra acquired from reference iron oxide samples.EELS analysis of the Fe 2p L2,3 edge experimentally acquired from pure samples of Fe3O4 (blue), γ-Fe2O3 (red) and α-Fe2O3 (black).
Mentions: The bright-field TEM image of Fig. 1b shows the same Fe3O4 grain after exposure to 9 mbar O2 atmosphere at 700 °C for 8 h within the ETEM. Degradation of the surface of the grain is apparent, while the associated SAED pattern (Fig. 1b, inset) does not present any evidence for the formation of additional crystalline phases, with brightening of some planar reflections (for example, the −2 8 6 reflection) being attributable to slight tilting of the grain during annealing. However, the development of fine features in the associated EEL spectrum of the heated grain, taking the form of a small pre-peak in the L3 edge and post-peak in the L2 edge (Fig. 1c, red arrows), is indicative of a change in the Fe oxidation state towards γ-Fe2O3 or α-Fe2O3 (refs 15, 16, 17), as illustrated by the reference iron oxide EEL spectra displayed in Fig. 2. It is recognized that progressive oxidation induces the development of these small peaks, with complete oxidation to γ-Fe2O3 being associated with an ~1.3 eV splitting in the L3 edge15. Similarly, the Fe 2p edge in an α-Fe2O3 EEL spectrum is associated with a strong pre-peak located ~1.6 eV in front of the L3 edge1617. Various iron hydroxides can also present similar pre-peaks in their Fe L3 edges; however, O2 is the only gas introduced into the system between acquisitions of EEL spectra, and hence the evolution of pre-peaks in this case is attributed solely to the effects of oxidation. The spacings between the central magnetic contours in the corresponding magnetic induction map (Fig. 1e), again flowing in a counterclockwise direction, were found to widen, most markedly towards the particle edge. Figure 1f,g presents magnetic contributions to the phase shifts used to construct Fig. 1d,e, respectively, and the reduction in amplitudes of the line profiles across their centres (dashed lines) is a strong indicator for loss of overall magnetic remanence in the Fe3O4 particle, as a consequence of oxidation (Fig. 1h, arrowed), again consistent with the progressive conversion of Fe3O4 towards γ-Fe2O3.

Bottom Line: Magnetite (Fe3O4) is an important magnetic mineral to Earth scientists, as it carries the dominant magnetic signature in rocks, and the understanding of its magnetic recording fidelity provides a critical tool in the field of palaeomagnetism.However, reliable interpretation of the recording fidelity of Fe3O4 particles is greatly diminished over time by progressive oxidation to less magnetic iron oxides, such as maghemite (γ-Fe2O3), with consequent alteration of remanent magnetization potentially having important geological significance.Here we use the complementary techniques of environmental transmission electron microscopy and off-axis electron holography to induce and visualize the effects of oxidation on the magnetization of individual nanoscale Fe3O4 particles as they transform towards γ-Fe2O3.

View Article: PubMed Central - PubMed

Affiliation: Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK.

ABSTRACT
Magnetite (Fe3O4) is an important magnetic mineral to Earth scientists, as it carries the dominant magnetic signature in rocks, and the understanding of its magnetic recording fidelity provides a critical tool in the field of palaeomagnetism. However, reliable interpretation of the recording fidelity of Fe3O4 particles is greatly diminished over time by progressive oxidation to less magnetic iron oxides, such as maghemite (γ-Fe2O3), with consequent alteration of remanent magnetization potentially having important geological significance. Here we use the complementary techniques of environmental transmission electron microscopy and off-axis electron holography to induce and visualize the effects of oxidation on the magnetization of individual nanoscale Fe3O4 particles as they transform towards γ-Fe2O3. Magnetic induction maps demonstrate a change in both strength and direction of remanent magnetization within Fe3O4 particles in the size range dominant in rocks, confirming that oxidation can modify the original stored magnetic information.

No MeSH data available.