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  • Journal article
    Ku J, Valdez-Grijalva M, Deng R, Zuo W, Chen Q, Lin H, Muxworthy Aet al., 2019,

    Modelling external magnetic fields of magnetite particles: From micro- to macro-scale

    , Geosciences, Vol: 9, ISSN: 2076-3263

    We determine the role of particle shape in the type of magnetic extraction processes used in mining. We use a micromagnetic finite element method (FEM) to analyze the effect of external magnetic fields on the magnetic structures of sub-micron magnetite particles. In non-saturating fields, the magnetite particles contain multiple possible non-uniform magnetization states. The non-uniformity was found to gradually disappear with increasing applied field strength; at 100 mT the domain structure became near uniform; at 300 mT the magnetic structure saturates and the magnetization direction aligned with the field. In magnetic separation techniques, we suggest that 100 mT is the optimal field for magnetite to maximize the magnetic field with the lowest energy transfer; larger particles, i.e., >1 µm, will likely saturate in smaller fields than this. We also examined the effect of external magnetic fields on a much larger irregular particle (L × W × H = 179.5 × 113 × 103 μm) that was too large to be examined using micromagnetics. To do this we used COMSOL. The results show the relative difference between the magnitude of magnetic flux density of the particle and that of a corresponding sphere of the same volume is <5% when the distance to the particle geometry center is more than five times the sphere radius. The ideas developed in this paper have the potential to improve magnetic mineral extraction yield.

  • Journal article
    Valdez-Grijalva MA, Muxworthy AR, 2019,

    First-order reversal curve (FORC) diagrams of nanomagnets with cubic magnetocrystalline anisotropy: a numerical approach

    , Journal of Magnetism and Magnetic Materials, Vol: 471, Pages: 359-364, ISSN: 0304-8853

    First-order reversal curve (FORC) diagrams are increasingly used as a material’s magnetic domain state fingerprint. FORC diagrams of noninteracting dispersions of single-domain (SD) particles with uniaxial magnetocrystalline anisotropy (MCA) are well studied. However, a large class of materials possess a cubic MCA, for which the FORC diagram properties of noninteracting SD particle dispersions are less understood. A coherent rotation model was implemented to study the FORC diagram properties of noninteracting ensembles of SD particles with positive and negative MCA constants. The pattern formation mechanism is identified and related to the irreversible events the individual particles undergo. Our results support the utility of FORC diagrams for the identification of noninteracting to weakly-interacting SD particles with cubic MCA.

  • Journal article
    Valdez-Grijalva MA, Muxworthy AR, Williams W, ó Conbhui P, Nagy L, Roberts AP, Heslop Det al., 2018,

    Magnetic vortex effects on first-order reversal curve (FORC) diagrams for greigite dispersions

    , Earth and Planetary Science Letters, Vol: 501, Pages: 103-111, ISSN: 0012-821X

    First-order reversal curve (FORC) diagrams are used increasingly in geophysics for magnetic domain state identification. The domain state of a magnetic particle is highly sensitive to particle size, about which FORC diagrams provide valuable information. However, the FORC signal of particles with nonuniform magnetisations, which are the main carrier of natural remanent magnetisations in many systems, is still poorly understood. In this study, the properties of non-interacting, randomly oriented dispersions of greigite (Fe3S4) in the uniform single-domain (SD) to non-uniform single-vortex (SV) size range are investigated via micromagnetic calculations. Signals for SD particles () are found to be in excellent agreement with previous SD coherent-rotation studies. A transitional range from to is identified for which a mixture of SD and SV behaviour produces complex FORC diagrams. Particles have purely SV behaviour with the remanent state for all particles in the ensemble in the SV state. It is found that for SV ensembles the FORC diagram provides a map of vortex nucleation and annihilation fields and that the FORC distribution peak should not be interpreted as the coercivity of the sample, but as a vortex annihilation field on the path to saturation.

  • Journal article
    Berndt TA, Chang L, Wang S, Badejo SAet al.,

    Time-Asymmetric FORC Diagrams: A New Protocol for Visualizing Thermal Fluctuations and Distinguishing Magnetic Mineral Mixtures

    , Geochemistry, Geophysics, Geosystems, ISSN: 1525-2027
  • Journal article
    Evans ME, Muxworthy AR, 2018,

    Vaalbara Palaeomagnetism

    , Canadian Journal of Earth Sciences, ISSN: 0008-4077

    Vaalbara is the name given to a proposed configuration of continental blocks—the Kaapvaal craton (southern Africa) and the Pilbara craton (north-western Australia)—thought to be the Earth’s oldest supercraton assemblage. Its temporal history is poorly defined, but it has been suggested that it was stable for at least 400 million years, between 3.1 and 2.7 Ga. Here, we present an updated analysis that shows that the existence of a single supercraton between ∼2.9 and ∼2.7 Ga is inconsistent with the available palaeomagnetic data.

  • Journal article
    Roberts AP, Zhao X, Harrison RJ, Heslop D, Muxworthy AR, Rowan CJ, Larrasoana J-C, Florindo Fet al., 2018,

    Signatures of Reductive Magnetic Mineral Diagenesis From Unmixing of First-Order Reversal Curves

    , JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, Vol: 123, Pages: 4500-4522, ISSN: 2169-9313
  • Journal article
    Harrison RJ, Muraszko J, Heslop D, Lascu I, Muxworthy AR, Roberts APet al., 2018,

    An Improved Algorithm for Unmixing First-Order Reversal Curve Diagrams Using Principal Component Analysis

    , GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, Vol: 19, Pages: 1595-1610, ISSN: 1525-2027
  • Journal article
    Ó Conbhuí P, Williams W, Fabian K, Muxworthy AR, Ridley P, Nagy Let al., 2018,

    MERRILL: micromagnetic earth related robust interpreted language laboratory

    , Geochemistry, Geophysics, Geosystems, Vol: 19, Pages: 1080-1106, ISSN: 1525-2027

    Complex magnetic domain structures and the energy barriers between them are responsiblefor pseudo-single-domain phenomena in rock magnetism and contribute significantly to the magneticremanence of paleomagnetic samples. This article introduces MERRILL, an open source software packagefor three-dimensional micromagnetics optimized and designed for the calculation of such complexstructures. MERRILL has a simple scripting user interface that requires little computational knowledge to usebut provides research strength algorithms to model complex, inhomogeneous domain structures inmagnetic materials. It uses a finite element/boundary element numerical method, optimally suited forcalculating magnetization structures of local energy minima (LEM) in irregular grain geometries that are ofinterest to the rock and paleomagnetic community. MERRILL is able to simulate the magnetic characteristicsof LEM states in both single grains, and small assemblies of interacting grains, including saddle-point pathsbetween nearby LEMs. Here the numerical model is briefly described, and an overview of the scriptinglanguage and available commands is provided. The open source nature of the code encourages futuredevelopment of the model by the scientific community.

  • Journal article
    Shah J, Williams W, Almeida TP, Nagy L, Muxworthy AR, Kovacs A, Valdez-Grijalva MA, Fabian K, Russell SS, Genge M, Dunin-Borkowski REet al., 2018,

    The oldest magnetic record in our solar system identified using nanometric imaging and numerical modeling

    , Nature Communications, Vol: 9, ISSN: 2041-1723

    Recordings of magnetic fields, thought to be crucial to our Solar System’s rapid accretion, are potentially retained in unaltered nanometric low-Ni kamacite (~metallic Fe) grains encased within dusty olivine crystals, found in the chondrules of unequilibrated chondrites. However, most of these kamacite grains are magnetically non-uniform, so their ability to retain four-billion-year-old magnetic recordings cannot be estimated by previous theories, which assume only uniform magnetization. Here, we demonstrate that non-uniformly magnetized nanometric kamacite grains are stable over Solar System timescales and likely the primary carrier of remanence in dusty olivine. By performing in-situ temperature-dependent nanometric magnetic measurements using off-axis electron holography, we demonstrate the thermal stability of multi-vortex kamacite grains from the chondritic Bishunpur meteorite. Combined with numerical micromagnetic modeling, we determine the stability of the magnetization of these grains. Our study shows that dusty olivine kamacite grains are capable of retaining magnetic recordings from the accreting Solar System.

  • Journal article
    Evans ME, Muxworthy AR, 2018,

    A re-appraisal of the proposed rapid Matuyama–Brunhes geomagnetic reversal in the Sulmona Basin, Italy

    , Geophysical Journal International, Vol: 213, Pages: 1744-1750, ISSN: 0956-540X

    An extremely sharp magnetic reversal observed in lacustrine sediments in central Italy has been interpreted as a record of the Matuyama-Brunhes geomagnetic polarity reversal that may represent less than a decade (Sagnotti et al., 2014. 2016). Here, we report new results from the same Sulmona Basin outcrop that question this interpretation. In particular, we find evidence of reversed (Matuyama) directions well above the proposed Matuyama-Brunhes Boundary (MBB). Coercivity spectra of anhysteretic remanent magnetization (ARM) imply a 3-component magnetic mineralogy: low-, intermediate-, and high-coercivity. The low-coercivity component is found in all but one of the samples and carries a strong modern overprint seen throughout the section. The high-coercivity component is dominated by volcanic material which is prone to remagnetization. Since it is much more magnetic than the surrounding lacustrine sediments, it may influence the remanence signal even when present at very low concentrations. The intermediate-coercivity component is the main carrier of any true primary remanence, but whether or not this can be isolated depends on the blocking-temperature and coercivity spectra of individual samples, and on the demagnetization method used. The complexity of the magnetization, the reversed zones above the proposed MBB, and the normal zones that Sagnotti and 2 colleagues found below it, lead to the conclusion that this section does not carry a reliable high-resolution record of the geomagnetic field. Thus, we feel that inferences about the stratigraphic position and duration of the MBB are premature.

  • 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, Vol: 122, Pages: 9534-9558, ISSN: 0148-0227

    The term “pseudo-single domain” (PSD) has been used to describe the transitional state in rockmagnetism 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 magneticmineral, magnetite, is so narrow (~20–75 nm) that it is widely considered that much of the paleomagneticrecord of interest is carried by PSD rather than stable SD particles. The PSD concept has, thus, become thedominant explanation for the magnetization associated with a major fraction of particles that recordpaleomagnetic signals throughout geological time. In this paper, we argue that in contrast to the SD and MDstates, the term PSD does not describe the relevant physical processes, which have been documentedextensively using three-dimensional micromagnetic modeling and by parallel research in material scienceand solid-state physics. We also argue that features attributed to PSD behavior can be explained bynucleation of a single magnetic vortex immediately above the maximum stable SD transition size. Withincreasing particle size, multiple vortices, antivortices, and domain walls can nucleate, which producevariable cancellation of magnetic moments and a gradual transition into the MD state. Thus, while the termPSD describes a well-known transitional state, it fails to describe adequately the physics of the relevantprocesses. We recommend that use of this term should be discontinued in favor of “vortex state,” whichspans a range of behaviors associated with magnetic vortices.

  • Journal article
    Valdez-Grijalva MA, Nagy L, Muxworthy AR, Williams W, Fabian Ket al., 2017,

    The magnetic structure and palaeomagnetic recording fidelity of sub-micron greigite (Fe3S4)

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

    We present the results of a finite-element micromagnetic model of View the MathML source to View the MathML source greigite (Fe3S4) 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 d0 is determined to be View the MathML source 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 View the MathML source 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 View the MathML source, when structures typical of MD behaviour like closure domains begin to form.

  • 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, Vol: 114, Pages: 13120-13125, ISSN: 0027-8424

    Nonideal, nonsingle-domain magnetic grains are ubiquitous in rocks; however, they can have a detrimental impact on the fidelity of paleomagnetic records—in particular the determination of ancient magnetic field strength (paleointensity), a key means of understanding the evolution of the earliest geodynamo and the formation of the solar system. As a consequence, great effort has been expended to link rock magnetic behavior to paleointensity results, but with little quantitative success. Using the most comprehensive rock magnetic and paleointensity data compilations, we quantify a stability trend in hysteresis data that characterizes the bulk domain stability (BDS) of the magnetic carriers in a paleomagnetic specimen. This trend is evident in both geological and archeological materials that are typically used to obtain paleointensity data and is therefore pervasive throughout most paleomagnetic studies. Comparing this trend to paleointensity data from both laboratory and historical experiments reveals a quantitative relationship between BDS and paleointensity behavior. Specimens that have lower BDS values display higher curvature on the paleointensity analysis plot, which leads to more inaccurate results. In-field quantification of BDS therefore reflects low-field bulk remanence stability. Rapid hysteresis measurements can be used to provide a powerful quantitative method for preselecting paleointensity specimens and postanalyzing previous studies, further improving our ability to select high-fidelity recordings of ancient magnetic fields. BDS analyses will enhance our ability to understand the evolution of the geodynamo and can help in understanding many fundamental Earth and planetary science questions that remain shrouded in controversy.

  • Journal article
    Almeida TP, Muxworthy AR, Kovacs A, Williams W, Dunin-Borkowski REet al., 2017,

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

    , Journal of Physics : Conference Series, Vol: 902, ISSN: 1742-6588

    A synthetic basalt comprising magnetic Fe3O4 grains (~ 50 nm to ~ 500 nm in diameter) is investigated using a range of complementary nano-characterisation techniques. Off-axis electron holography combined with in situ heating allowed for the visualisation of the thermally-induced magnetic relaxation of an Fe3O4 grain (~ 300 nm) from an irregular domain state into a vortex state at 550˚C, just below its Curie temperature, with the magnetic intensity of the vortex increasing on cooling.

  • Journal article
    Muxworthy AR, 2017,

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

    , Frontiers in Earth Science, ISSN: 2296-6463

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