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  • JOURNAL ARTICLE
    Valdez-Grijalva MA, Nagy L, Muxworthy AR, Williams W, Fabian Ket al., 2018,

    The magnetic structure and palaeomagnetic recording fidelity of sub-micron greigite (Fe<inf>3</inf>S<inf>4</inf>)

    , Earth and Planetary Science Letters, Vol: 483, Pages: 76-89, ISSN: 0012-821X

    © 2017 The Authors We present the results of a finite-element micromagnetic model of 30nm to 300nm greigite (Fe 3 S 4 ) grains with a variety of equant morphologies. This grain size range covers the magnetic single-domain (SD) to pseudo single-domain (PSD) transition, and possibly also the PSD to multi-domain (MD) transition. The SD–PSD threshold d 0 is determined to be 50nm≤d 0 ≤56nm depending on grain shape. The nudged elastic-band method was used to determine the room temperature energy barriers between stable states and thus the blocking volumes. It is found that, in the absence of interparticle magnetostatic interactions, the magnetisation of equant SD greigite is not stable on a geological scale and only PSD grains ≥70nm can be expected to carry a stable magnetisation over billion-year timescales, i.e., all non-interacting SD particles are essentially superparamagnetic. We further identify a mechanism for the PSD to multi-domain (MD) transition, which is of a continuous nature from PSD nucleation up to 300nm, when structures typical of MD behaviour like closure domains begin to form.

  • JOURNAL ARTICLE
    Almeida TP, Muxworthy AR, Kovcas A, Williams W, Dunin-Borkowski REet al., 2017,

    Observation of thermally-induced magnetic relaxation in a magnetite grain using off-axis electron holography

    , ELECTRON MICROSCOPY AND ANALYSIS GROUP CONFERENCE 2017 (EMAG2017), Vol: 902, ISSN: 1742-6588
  • JOURNAL ARTICLE
    Berndt T, Muxworthy AR, 2017,

    Dating Icelandic glacial floods using a new viscous remanent magnetization protocol

    , GEOLOGY, Vol: 45, Pages: 339-342, ISSN: 0091-7613
  • JOURNAL ARTICLE
    Berndt T, Paterson GA, Cao C, Muxworthy ARet al., 2017,

    Experimental test of the heating and cooling rate effect on blocking temperatures

    , GEOPHYSICAL JOURNAL INTERNATIONAL, Vol: 210, Pages: 255-269, ISSN: 0956-540X
  • JOURNAL ARTICLE
    Berndt T, Ramalho RS, Valdez-Grijalva MA, Muxworthy ARet al., 2017,

    Paleomagnetic field reconstruction from mixtures of titanomagnetites

    , Earth and Planetary Science Letters, Vol: 465, Pages: 70-81, ISSN: 0012-821X

    Stepwise thermal demagnetization and alternating field (AF) demagnetization are commonly used in paleomagnetic studies to isolate remanent magnetic components of different origins. The magnetically hardest, i.e. highest unblocking temperature/peak field component is often interpreted as the primary magnetization and magnetically softer components as subsequent remagnetizations due to geological events posterior to the formation of the rock, such as reheating or formation of new magnetic minerals. The correct interpretation of the sequence of the geological events such as tectonic rotations from paleomagnetic data often relies on correctly attributing the observed magnetic directions to the remanence carriers and acquisition mechanisms. Using a numerical model to simulate remanence acquisition and stepwise thermal and AF demagnetization experiments, we show that the presence of mixtures of different magnetic minerals, such as magnetite and titanomagnetites of varying titanium-content can have very significant effects on Zijderveld plots. In thermal demagnetization experiments a spurious third component at intermediate temperatures or a continuous curvature may arise from an overlap of the primary remanence with a subsequent thermal or viscous remagnetization carried by small-grained iron-rich magnetite and large-grained titanium-rich titanomagnetite. AF demagnetization plots of magnetic mixtures are even more complex: primary and secondary remanences carried by different minerals may appear as either three or four components in Zijderveld plots. During alternating field demagnetization the highest coercivity component is not necessarily equivalent to the primary remanence and does not necessarily correspond to the highest temperature component in an analogous thermal demagnetization experiment, i.e., the primary remanence direction cannot be recovered. The effects are shown to be due to the different responsiveness of magnetite and titanomagnetites towards viscous or

  • JOURNAL ARTICLE
    Di Chiara A, Muxworthy AR, Trindade RIF, Bispo-Santos Fet al., 2017,

    Paleoproterozoic Geomagnetic Field Strength From the Avanavero Mafic Sills, Amazonian Craton, Brazil

    , GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, Vol: 18, Pages: 3891-3903, ISSN: 1525-2027
  • JOURNAL ARTICLE
    Hofman J, Maher BA, Muxworthy AR, Wuyts K, Castanheiro A, Samson Ret al., 2017,

    Biomagnetic Monitoring of Atmospheric Pollution: A Review of Magnetic Signatures from Biological Sensors

    , ENVIRONMENTAL SCIENCE & TECHNOLOGY, Vol: 51, Pages: 6648-6664, ISSN: 0013-936X
  • BOOK CHAPTER
    Maidment S, Balikova D, Muxworthy AR, 2017,

    Magnetostratigraphy of the Morrison Formation, Utah

    , Terrestrial Depositional Systems Deciphering Complexities Through Multiple Stratigraphic Methods, Editors: Zeigler, Parker, Publisher: Elsevier, ISBN: 9780128032435

    Terrestrial Depositional Systems: Deciphering Complexities through Multiple Stratigraphic Methods is the first collection of contributed articles that not only introduces young geoscientists to biostratigraphy, chemostratigraphy, ...

  • JOURNAL ARTICLE
    Muxworthy AR, 2017,

    Considerations for latitudinal time-averaged-field palaeointensity analysis of the last five million years

    , Frontiers in Earth Science, Vol: 5

    © 2017 Muxworthy. Central to palaeomagnetism and geophysics is the assumption that the time-averaged geomagnetic field is approximated by a geocentric-axial-dipole (GAD). In this paper, it is demonstrated through the use of a simple cap model that due to secular variation the time-averaged palaeointensity record will always have a smaller latitudinal dependency than a true GAD field. However, the simple cap model does not fully explain the behavior of the palaeointensity database (averaged over 0–5Ma) especially at high-latitudes. To investigate this dependency I use a Giant Gaussian Processes (GGP) model to estimate the contribution of permanent non-dipole features and determine their statistical significance. It was found that an axial quadrupole term between −5 and −10% of the GAD field combined with octupole term ∼ −15% of the GAD field, best explained palaeointensity latitudinal behavior. In particular, the octupole term with a sign opposite to that of the GAD, is required to describe the palaeointensity behavior at high latitudes, i.e., > 60°.

  • JOURNAL ARTICLE
    Muxworthy AR, Bland PA, Davison TM, Moore J, Collins GS, Ciesla FJet al., 2017,

    Evidence for an impact-induced magnetic fabric in Allende, and exogenous alternatives to the core dynamo theory for Allende magnetization

    , METEORITICS & PLANETARY SCIENCE, Vol: 52, Pages: 2132-2146, ISSN: 1086-9379
  • JOURNAL ARTICLE
    Nagy L, Williams W, Muxworthy AR, Fabian K, Almeida TP, Conbhui PO, Shcherbakov VPet al., 2017,

    Stability of equidimensional pseudo-single-domain magnetite over billion-year timescales

    , PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, Vol: 114, Pages: 10356-10360, ISSN: 0027-8424
  • JOURNAL ARTICLE
    Paterson GA, Muxworthy AR, Yamamoto Y, Pan Yet al., 2017,

    Bulk magnetic domain stability controls paleointensity fidelity

    , PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, Vol: 114, Pages: 13120-13125, ISSN: 0027-8424
  • JOURNAL ARTICLE
    Roberts AP, Almeida TP, Church NS, Harrison RJ, Heslop D, Li Y, Li J, Muxworthy AR, Williams W, Zhao Xet al., 2017,

    Resolving the Origin of Pseudo-Single Domain Magnetic Behavior

    , Journal of Geophysical Research: Solid Earth, ISSN: 2169-9313

    © 2017. American Geophysical Union. All Rights Reserved. The term "pseudo-single domain" (PSD) has been used to describe the transitional state in rock magnetism that spans the particle size range between the single domain (SD) and multidomain (MD) states. The particle size range for the stable SD state in the most commonly occurring terrestrial magnetic mineral, magnetite, is so narrow (~20-75 nm) that it is widely considered that much of the paleomagnetic record of interest is carried by PSD rather than stable SD particles. The PSD concept has, thus, become the dominant explanation for the magnetization associated with a major fraction of particles that record paleomagnetic signals throughout geological time. In this paper, we argue that in contrast to the SD and MD states, the term PSD does not describe the relevant physical processes, which have been documented extensively using three-dimensional micromagnetic modeling and by parallel research in material science and solid-state physics. We also argue that features attributed to PSD behavior can be explained by nucleation of a single magnetic vortex immediately above the maximum stable SD transition size. With increasing particle size, multiple vortices, antivortices, and domain walls can nucleate, which produce variable cancellation of magnetic moments and a gradual transition into the MD state. Thus, while the term PSD describes a well-known transitional state, it fails to describe adequately the physics of the relevant processes. We recommend that use of this term should be discontinued in favor of "vortex state," which spans a range of behaviors associated with magnetic vortices.

  • JOURNAL ARTICLE
    Shah J, Bates HC, Muxworthy AR, Hezel DC, Russell SS, Genge MJet al., 2017,

    Long-lived magnetism on chondrite parent bodies

    , EARTH AND PLANETARY SCIENCE LETTERS, Vol: 475, Pages: 106-118, ISSN: 0012-821X
  • JOURNAL ARTICLE
    Wilkinson JJ, Vowles K, Muxworthy AR, Mac Niocaill Cet al., 2017,

    Regional remagnetization of Irish Carboniferous carbonates dates Variscan orogenesis, not Zn-Pb mineralization

    , GEOLOGY, Vol: 45, Pages: 747-750, ISSN: 0091-7613

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