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Journal articleNagy L, Williams W, Tauxe L, et al., 2019,
From nano to micro: evolution of magnetic domain structures in multi‐domain magnetite
, Geochemistry, Geophysics, Geosystems, Vol: 20, Pages: 2907-2918, ISSN: 1525-2027Reliability of magnetic recordings of the ancient magnetic field is strongly dependent on the magnetic mineralogy of natural samples. Theoretical estimates of long‐term stability of remanence were restricted to single‐domain (SD) states, but micromagnetic models have recently demonstrated that the so‐called single‐vortex (SV) domain structure can have even higher stability that SD grains. In larger grains ( urn:x-wiley:ggge:media:ggge21913:ggge21913-math-000110 μm in magnetite) the multidomain (MD) state dominates, so that large uniform magnetic domains are separated by narrow domain walls. In this paper we use a parallelized micromagnetic finite element model to provide resolutions of many millions of elements allowing us, for the first time, to examine the evolution of magnetic structure from a uniform state, through the SV state up to the development of the domain walls indicative of MD states. For a cuboctahedral grain of magnetite, we identify clear domain walls in grains as small as ∼3 μm with domain wall widths equal to that expected in large MD grains; we therefore put the SV to MD transition at ∼3 μm for magnetite and expect well‐defined, and stable, SV structures to be present until at least ∼1 μm when reducing the grain size. Reducing the size further shows critical dependence on the history of domain structures, particularly with SV states that transition through a so‐called “unstable zone” leading to the recently observed hard‐aligned SV states that proceed to unwind to SD yet remain hard aligned.
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Journal articlePenny C, Muxworthy A, Fabian K, 2019,
Mean-field modelling of magnetic nanoparticles: The effect of particle size and shape on the Curie temperature
, Physical review B: Condensed matter and materials physics, Vol: 99, ISSN: 1098-0121A Heisenberg mean-field model is used to study the effect of size and shape on the Curie temperature of magnetic nanoparticles. Simple cubic, body-centered cubic, and magnetite nanoparticles are modelled as spheres, cubes, and needlelike particles. The Curie temperatures of particles of different shape, but with the same crystal structure and smallest dimension d, are found to differ. The range in the value of the Curie temperature between particles of different shape, ΔTC, is found to be ∼20% of the bulk value of TC in particles where d<10 atoms. As particle size increases, the value of ΔTC reduces rapidly and becomes negligible above a threshold size. This threshold size differs between systems and is controlled predominantly by crystal structure. All systems were fit to the finite-size scaling equation, with values of the scaling exponent ν found to lie between 0.46 and 0.55, in good agreement with the expected value of ν=0.5. No trend in the value of ν due to shape was found.
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Journal articleKu J, Valdez-Grijalva M, Deng R, et al., 2019,
Modelling external magnetic fields of magnetite particles: From micro- to macro-scale
, Geosciences, Vol: 9, ISSN: 2076-3263We 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.
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Journal articleValdez-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: 0304-8853First-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.
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Journal articleNagy L, Williiams W, Tauxe L, et al., 2018,
Thermomagnetic recording fidelity of nanometer sized iron: implications for planetary magnetism
, Proceedings of the National Academy of Sciences, ISSN: 0027-8424 -
Conference paperKu J, Liu X, Muxworthy AR, et al., 2018,
Simplified modelling of particle magnetic induction field based on FEM
, AMICR 2018, Publisher: WILEY, Pages: 54-55, ISSN: 1742-7835 -
Journal articleValdez-Grijalva MA, Muxworthy AR, Williams W, et 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: 0012-821XFirst-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.
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Journal articleEvans ME, Muxworthy AR, 2018,
Vaalbara Palaeomagnetism
, Canadian Journal of Earth Sciences, Vol: 56, Pages: 912-916, ISSN: 0008-4077Vaalbara 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.
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Journal articleZhang Y, Jia D, Muxworthy AR, et al., 2018,
The Chemical Remagnetization of Ediacaran Dolomite in the Taishan Paleo-Reservoir, South China
, JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, Vol: 123, Pages: 6161-6175, ISSN: 2169-9313 -
Journal articleRoberts AP, Zhao X, Harrison RJ, et 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 articleHarrison RJ, Muraszko J, Heslop D, et 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 articleÓ Conbhuí P, Williams W, Fabian K, et al., 2018,
MERRILL: micromagnetic earth related robust interpreted language laboratory
, Geochemistry, Geophysics, Geosystems, Vol: 19, Pages: 1080-1106, ISSN: 1525-2027Complex 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.
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Journal articleShah J, Williams W, Almeida TP, et al., 2018,
The oldest magnetic record in our solar system identified using nanometric imaging and numerical modeling
, Nature Communications, Vol: 9, ISSN: 2041-1723Recordings 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.
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Journal articleEvans 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: 0956-540XAn 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.
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Journal articleRoberts AP, Almeida TP, Church NS, et al., 2017,
Resolving the origin of pseudo-single domain magnetic behavior
, Journal of Geophysical Research, Vol: 122, Pages: 9534-9558, ISSN: 0148-0227The 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.
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