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  • Journal article
    Berndt T, Muxworthy AR, Paterson GA, 2015,

    Determining the magnetic attempt time τ0, its temperature dependence and the grain-size distribution from magnetic viscosity measurements

    , Journal of Geophysical Research. Solid Earth, Vol: 120, Pages: 7322-7336, ISSN: 2169-9313

    A new method to determine the atomic attempt time τ0 of magnetic relaxation of fine particles, which is central to rock and soil magnetism and paleomagnetic recording theory, is presented, including the determination of its temperature dependence, and simultaneously the grain-size distribution of a sample. It is based on measuring a series of zero-field magnetic viscous decay curves for saturation isothermal remanent magnetization at various different temperatures, that are later joined together on a single grain-size scale from which the grain-size distribution and attempt time are determined. The attempt time was determined for three samples containing non-interacting, single-domain titanomagnetites of different grain-sizes for temperatures between 27 K and 374 K. No clear temperature-dependent trend was found, however, values varied significantly from one sample to the other: from 10⁻¹¹ s to 10⁻⁸ s; in particular, the sample containing multiple magnetic phases had an effective attempt time significantly lower than the more homogeneous samples, thereby questioning the applicability of the simple Néel-Arhennius equation for magnetic relaxation for composite materials.

  • Journal article
    Almeida TP, Muxworthy AR, Kovács A, Williams W, Dunin-Borkowski REet al., 2015,

    Visualisation of high temperature magnetisation states in magnetite grains using off-axis electron holography

    , Journal of Physics: Conference Series, ISSN: 1742-6588
  • Journal article
    Abubakar R, Muxworthy AR, Southern P, Watson JS, Fraser AJ, Almeida TP, Sephton MAet al., 2015,

    Formation of magnetic minerals in hydrocarbon-generation conditions

    , Marine and Petroleum Geology, ISSN: 1873-4073

    In this paper, we report the pyrolysis and formation of magnetic minerals in three source rock samples from the Wessex Basin in Dorset, southern England. The experimental conditions in the laboratory recreated the catagenesis environment of oil source rocks. Magnetic analysis of both the heated and the unheated samples at room temperature and at very low temperatures (5 K), coupled with transmission electron-microscopy imaging and X-ray analysis, revealed the formation of nanometre-sized (<10 nm), magnetic particles that varied across the rock samples analysed, but more importantly across the pyrolysis temperature range. Magnetic measurements demonstrated the formation of these magnetic minerals peaked at 250 °C for all rock samples and then decreased at 300 °C before rising again at 320 °C. The newly formed magnetic minerals are suggested to be primarily pyrrhotite, though magnetite and greigite are also thought to be present. The sizes of the magnetic minerals formed suggest a propensity to migrate together with oil potentially explaining the magnetic anomalies observed above and within oil fields.

  • Conference paper
    Abubakar R, Muxworthy AR, Sephton M, Fraser A, Heslop D, Paterson G, Southern Pet al., 2015,

    Mapping Petroluem Migration Pathways Using Magnetics and Seismic Interpretations (poster)

    , AGU Fall 2015
  • Conference paper
    Muxworthy AR, 2015,

    How well does Preisach Theory predict Pseudo-Single-Domain Behavior? (poster)

    , AGU Fall 2015
  • Journal article
    Muxworthy AR, 2015,

    Investigation of magnetic particulate matter inside animals' lung tissue: preliminary results

    , STUDIA GEOPHYSICA ET GEODAETICA, Vol: 59, Pages: 628-634, ISSN: 0039-3169
  • Journal article
    Almeida TP, Muxworthy AR, Kasama T, Williams W, Damsgaard C, Frandsen C, Pennycook TJ, Dunin-Borkowski REet al., 2015,

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

    , Geochemistry Geophysics Geosystems, Vol: 16, Pages: 2969-2979, ISSN: 1525-2027

    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 confirms the precursor grains as Fe3O4 ranging from ~ 150 nm 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 ºC to investigate their oxidation to maghemite (γ-Fe2O3). High-angle annular dark field imaging and localized electron energy loss spectroscopy reveals slightly oxidized Fe3O4 grains, heated to 140 ºC, 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 ºC, but subsequently decreases after heating above 200 ºC; 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.

  • Conference paper
    Shah J, Muxworthy AR, Almeida TP, Kovacs A, Russell SS, Genge MJ, Dunin-Borkowski REet al., 2015,

    VISUALIZING THE MAGNETIC BEHAVIOR OF CHONDRULE DUSTY OLIVINE USING ELECTRON HOLOGRAPHY.

    , 78th Annual Meeting of the Meteoritical-Society, Publisher: WILEY-BLACKWELL, ISSN: 1086-9379
  • Conference paper
    Shah J, Muxworthy AR, Russell SS, Genge MJet al., 2015,

    USING MICRO-CT TO MAP METEORITIC MAGNETISM.

    , 78th Annual Meeting of the Meteoritical-Society, Publisher: WILEY-BLACKWELL, ISSN: 1086-9379
  • Conference paper
    Muxworthy AR, Bland PA, Collins G, Moore Jet al., 2015,

    MAGNETIC FABRICS IN ALLENDE: IMPLICATIONS FOR MAGNETIC REMANENCE ACQUISITION.

    , 78th Annual Meeting of the Meteoritical-Society, Publisher: WILEY, ISSN: 1086-9379
  • Journal article
    Muxworthy AR, Williams W, 2015,

    Critical single-domain grain sizes in elongated iron particles: implications for meteoritic and lunar magnetism

    , Geophysical Journal International, Vol: 202, Pages: 578-583, ISSN: 1365-246X

    Kamacite particles (Fe–Ni, Ni < 5 per cent), are very common in extra-terrestrial materials, such as meteorites. It is normally assumed that for kamacite particles to be reliable recorders of magnetic fields, they need to be magnetically uniform (single domain, SD) and thermally stable. Larger particles subdivide into non-uniform multidomain (MD) magnetic structures that produce weaker magnetic signals, while small SD particles become magnetically unstable due to thermal fluctuations and exhibit superparamagnetic behaviour. In this paper we determine the first micromagnetic calculation of the stable SD range domain-state phase diagram for metallic iron; previous calculations were analytical. There is a significant increase in the critical size for the SD/MD threshold size, for example, for cube-shaped iron particles, the critical SD/MD threshold has now been estimated to be 25 nm, compared to 17 nm for previous estimates. The larger critical SD/MD threshold size for iron, agrees better with previously published nanometric observations of domain state for FeNi particles, then early analytical models.

  • Conference paper
    Berndt T, Muxworthy AR, Paterson G, van Ginneken Met al., 2015,

    Temperature and grain size dependence of the attempt time t0 for fine magnetic particles

    , IUGG 2015
  • Conference paper
    Di Chiara A, Muxworthy AR, Trindade R, 2015,

    Palaeointensity of Proterozoic magmatic rocks from South America using the Preisach method, preliminary results

    , IUGG 2015
  • Conference paper
    Berndt T, Muxworthy AR, 2015,

    Viscous remanent magnetization dating of cataclysmic floods in Iceland

    , IUGG 2015
  • Conference paper
    Døssing A, Muxworthy AR, Mac Niocaill C, Riishuus Met al., 2015,

    Geomagnetic secular variation during 6-0.5 Ma: Paleomagnetic results from Eastern Iceland

    , IUGG 2015

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