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

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          Abstract

          Magnetite (Fe 3O 4) 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 Fe 3O 4 particles is greatly diminished over time by progressive oxidation to less magnetic iron oxides, such as maghemite (γ-Fe 2O 3), 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 Fe 3O 4 particles as they transform towards γ-Fe 2O 3. Magnetic induction maps demonstrate a change in both strength and direction of remanent magnetization within Fe 3O 4 particles in the size range dominant in rocks, confirming that oxidation can modify the original stored magnetic information.

          Abstract

          Magnetite provides a valuable record of the Earth's geomagnetic history. Here, Almeida et al. combine electron microscopy and energy-loss spectroscopy to study the effects of in situ oxidation on the magnetization fidelity and crystalline phase of pseudo-single domain magnetite grains.

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          Magnetic microstructure of magnetotactic bacteria by electron holography

          Off-axis electron holography in the transmission electron microscope was used to correlate the physical and magnetic microstructure of magnetite nanocrystals in magnetotactic bacteria. The magnetite crystals were all single magnetic domains, and the magnetization directions of small superparamagnetic crystals were constrained by magnetic interactions with larger crystals in the chains. Shape anisotropy was found to dominate magnetocrystalline anisotropy in elongated crystals. A coercive field between 300 and 450 oersted was determined for one chain.
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            Direct imaging of nanoscale magnetic interactions in minerals.

            The magnetic microstructure of a natural, finely exsolved intergrowth of submicron magnetite blocks in an ulvöspinel matrix is characterized by using off-axis electron holography in the transmission electron microscope. Single-domain and vortex states in individual blocks, as well as magnetostatic interaction fields between them, are imaged at a spatial resolution approaching the nanometer scale. The images reveal an extremely complicated magnetic structure dominated by the shapes of the blocks and magnetostatic interactions. Magnetic superstates, in which clusters of magnetite blocks act collectively to form vortex and multidomain states that have zero net magnetization, are observed directly.
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              Improving energy resolution of EELS spectra: an alternative to the monochromator solution.

              In this paper, we propose a numerical method which can routinely improve the energy resolution down to 0.2-0.3eV of electron energy-loss spectra acquired in a transmission electron microscope. The method involves measurement of the point-spread function (PSF) corresponding to the spectrometer aberration and to the incident energy spread, and then an inversion of this PSF so as to restore the spectrum. The chosen algorithm is based on an iterative calculation of the maximum likelihood solution known to be very robust against small errors in the PSF used. Restorations have been performed on diamond and graphite C-K edges acquired with an initial energy resolution of around 1eV. After reconstruction, the sharp core exciton lines become clearly visible for both compounds and the final energy resolution is estimated to be about 200-300meV. In the case of graphite, restorations involving both energy resolution and angular resolution have been successfully conducted. Finally, restorations of Fe L(2,3) and O-K edges measured for various iron oxides will be shown.
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                Author and article information

                Journal
                Nat Commun
                Nat Commun
                Nature Communications
                Nature Pub. Group
                2041-1723
                10 October 2014
                : 5
                : 5154
                Affiliations
                [1 ]Department of Earth Science and Engineering, Imperial College London , South Kensington Campus, London SW7 2AZ, UK
                [2 ]Center for Electron Nanoscopy, Technical University of Denmark , Kongens Lyngby DK-2800, Denmark
                [3 ]School of GeoSciences, University of Edinburgh, The King’s Buildings , West Mains Road, Edinburgh EH9 3JW, UK
                [4 ]Division of Materials, Mechanics and Structures, Department of Mechanical, Materials and Manufacturing Engineering, Faculty of Engineering, University of Nottingham, University Park , Nottingham NG7 2RD, UK
                [5 ]Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg Institute, Forschungszentrum Jülich , Jülich D-52425, Germany
                Author notes
                Article
                ncomms6154
                10.1038/ncomms6154
                4214405
                25300366
                a4de98b8-c8d7-455a-a540-478d02fb2a8b
                Copyright © 2014, Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.

                This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

                History
                : 13 May 2014
                : 04 September 2014
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