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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.


Visualized effect of oxidation on the magnetization of an equiaxed Fe3O4 particle.Bright-field TEM images acquired (a) before and (b) after in situ heating to 700 °C under 9 mbar of O2 for 8 h in an ETEM, with associated SAED patterns inset, indexed to Fe3O4 (Joint Committee on Powder Diffraction Standards (JCPDS) No. 75–449). (c) Associated EEL spectra of the Fe 2p L2,3 edge acquired from the Fe3O4 particle before (blue) and after (red) annealing within the ETEM. Black arrows emphasize three differing intensities from the mixed-valence compound of Fe3O4, while the red arrows highlight formation of pre- and post-peaks that indicate oxidation towards γ-Fe2O3. (d,e) Magnetic induction maps determined from the magnetic contribution to the phase shift, reconstructed from holograms taken (d) before and (e) after in situ heating, revealing the vortex nature of the particle. The contour spacing is 0.79 radians for both magnetic induction maps. The magnetization direction is shown using arrows, as depicted in the colour wheel. (f,g) Magnetic contributions to the phase shift, as used to reconstruct the magnetic induction maps in (d,e), respectively, and (h) line profiles across their centers before (blue) and after (red) annealing. Black arrows in h illustrate the loss in overall magnetic remanence. Scale bars represent 100 nm.
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f1: Visualized effect of oxidation on the magnetization of an equiaxed Fe3O4 particle.Bright-field TEM images acquired (a) before and (b) after in situ heating to 700 °C under 9 mbar of O2 for 8 h in an ETEM, with associated SAED patterns inset, indexed to Fe3O4 (Joint Committee on Powder Diffraction Standards (JCPDS) No. 75–449). (c) Associated EEL spectra of the Fe 2p L2,3 edge acquired from the Fe3O4 particle before (blue) and after (red) annealing within the ETEM. Black arrows emphasize three differing intensities from the mixed-valence compound of Fe3O4, while the red arrows highlight formation of pre- and post-peaks that indicate oxidation towards γ-Fe2O3. (d,e) Magnetic induction maps determined from the magnetic contribution to the phase shift, reconstructed from holograms taken (d) before and (e) after in situ heating, revealing the vortex nature of the particle. The contour spacing is 0.79 radians for both magnetic induction maps. The magnetization direction is shown using arrows, as depicted in the colour wheel. (f,g) Magnetic contributions to the phase shift, as used to reconstruct the magnetic induction maps in (d,e), respectively, and (h) line profiles across their centers before (blue) and after (red) annealing. Black arrows in h illustrate the loss in overall magnetic remanence. Scale bars represent 100 nm.

Mentions: Figure 1 illustrates the effect of accelerated oxidation on the magnetization of an individual, equiaxed synthetic Fe3O4 grain, as assessed using TEM, selected area electron diffraction (SAED) and EELS, along with associated magnetic induction maps. The bright-field TEM image of Fig. 1a shows a native, smooth-surfaced, ~200 nm diameter Fe3O4 grain, as indicated by SAED (Fig. 1a, inset). EELS analysis of the Fe 2p L2,3 edge, in the region 704–726 eV (Fig. 1c), confirmed the assignment of pure Fe3O4. The L2 edge for this sample shows the typical shape of a mixed-valence compound, that is, three visible features of differing intensities (Fig. 1c, black arrows), while the almost-shapeless L3 edge is attributed to the combined spectral contributions of different iron sites (that is, Fe2+ at octahedral B-sites and Fe3+ at both tetrahedral A and octahedral B-sites), consistent with the more delocalized structure of Fe3O4, as compared with other mixed iron oxides1314. The corresponding magnetic induction map of Fig. 1d exhibits evenly spaced magnetic contours, spanning from the surface to the centre of the grain, flowing in a counterclockwise direction (denoted by arrows), characteristic of a vortex state.


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)

Visualized effect of oxidation on the magnetization of an equiaxed Fe3O4 particle.Bright-field TEM images acquired (a) before and (b) after in situ heating to 700 °C under 9 mbar of O2 for 8 h in an ETEM, with associated SAED patterns inset, indexed to Fe3O4 (Joint Committee on Powder Diffraction Standards (JCPDS) No. 75–449). (c) Associated EEL spectra of the Fe 2p L2,3 edge acquired from the Fe3O4 particle before (blue) and after (red) annealing within the ETEM. Black arrows emphasize three differing intensities from the mixed-valence compound of Fe3O4, while the red arrows highlight formation of pre- and post-peaks that indicate oxidation towards γ-Fe2O3. (d,e) Magnetic induction maps determined from the magnetic contribution to the phase shift, reconstructed from holograms taken (d) before and (e) after in situ heating, revealing the vortex nature of the particle. The contour spacing is 0.79 radians for both magnetic induction maps. The magnetization direction is shown using arrows, as depicted in the colour wheel. (f,g) Magnetic contributions to the phase shift, as used to reconstruct the magnetic induction maps in (d,e), respectively, and (h) line profiles across their centers before (blue) and after (red) annealing. Black arrows in h illustrate the loss in overall magnetic remanence. Scale bars represent 100 nm.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Visualized effect of oxidation on the magnetization of an equiaxed Fe3O4 particle.Bright-field TEM images acquired (a) before and (b) after in situ heating to 700 °C under 9 mbar of O2 for 8 h in an ETEM, with associated SAED patterns inset, indexed to Fe3O4 (Joint Committee on Powder Diffraction Standards (JCPDS) No. 75–449). (c) Associated EEL spectra of the Fe 2p L2,3 edge acquired from the Fe3O4 particle before (blue) and after (red) annealing within the ETEM. Black arrows emphasize three differing intensities from the mixed-valence compound of Fe3O4, while the red arrows highlight formation of pre- and post-peaks that indicate oxidation towards γ-Fe2O3. (d,e) Magnetic induction maps determined from the magnetic contribution to the phase shift, reconstructed from holograms taken (d) before and (e) after in situ heating, revealing the vortex nature of the particle. The contour spacing is 0.79 radians for both magnetic induction maps. The magnetization direction is shown using arrows, as depicted in the colour wheel. (f,g) Magnetic contributions to the phase shift, as used to reconstruct the magnetic induction maps in (d,e), respectively, and (h) line profiles across their centers before (blue) and after (red) annealing. Black arrows in h illustrate the loss in overall magnetic remanence. Scale bars represent 100 nm.
Mentions: Figure 1 illustrates the effect of accelerated oxidation on the magnetization of an individual, equiaxed synthetic Fe3O4 grain, as assessed using TEM, selected area electron diffraction (SAED) and EELS, along with associated magnetic induction maps. The bright-field TEM image of Fig. 1a shows a native, smooth-surfaced, ~200 nm diameter Fe3O4 grain, as indicated by SAED (Fig. 1a, inset). EELS analysis of the Fe 2p L2,3 edge, in the region 704–726 eV (Fig. 1c), confirmed the assignment of pure Fe3O4. The L2 edge for this sample shows the typical shape of a mixed-valence compound, that is, three visible features of differing intensities (Fig. 1c, black arrows), while the almost-shapeless L3 edge is attributed to the combined spectral contributions of different iron sites (that is, Fe2+ at octahedral B-sites and Fe3+ at both tetrahedral A and octahedral B-sites), consistent with the more delocalized structure of Fe3O4, as compared with other mixed iron oxides1314. The corresponding magnetic induction map of Fig. 1d exhibits evenly spaced magnetic contours, spanning from the surface to the centre of the grain, flowing in a counterclockwise direction (denoted by arrows), characteristic of a vortex state.

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.