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  • Articles  (29)
  • Wiley  (25)
  • Geological Society of London  (3)
  • American Institute of Physics  (1)
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  • Articles  (29)
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  • 1
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    Direct observation of the thermal demagnetization of magnetic vortex structures in non-ideal magnetite recorders (2016)
    Wiley
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    Publication Date: 2016-07-31
    Description: The thermal demagnetization of pseudo-single-domain (PSD) magnetite (Fe 3 O 4 ) particles, which govern the magnetic signal in many igneous rocks, is examined using off-axis electron holography. Visualization of a vortex structure held by an individual Fe 3 O 4 particle (~250 nm in diameter) during in situ heating is achieved through the construction and examination of magnetic-induction maps. Step-wise demagnetization of the remanence-induced Fe 3 O 4 particle upon heating to above the Curie temperature, performed in a similar fashion to bulk thermal demagnetization measurements, revealed its vortex state remains stable under heating close to its unblocking temperature, and is recovered upon cooling with the same or reversed vorticity. Hence, the PSD Fe 3 O 4 particle exhibits thermomagnetic behavior comparable to a single-domain carrier, and thus vortex-states are considered reliable magnetic recorders for paleomagnetic investigations.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Effect of maghemization on the magnetic properties of nonstoichiometric pseudosingle-domain magnetite particles (2015)
    Wiley
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    Publication Date: 2015-08-07
    Description: The effect of maghemization on the magnetic properties of magnetite (Fe 3 O 4 ) 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 Fe 3 O 4 ranging from ∼ 150 nm to ∼ 250 nm in diameter, whilst Mössbauer spectrometry suggests the grains are initially near-stoichiometric. The Fe 3 O 4 grains are heated to increasing reaction temperatures of 120 – 220 ºC to investigate their oxidation to maghemite (γ-Fe 2 O 3 ). High-angle annular dark field imaging and localized electron energy-loss spectroscopy reveals slightly oxidized Fe 3 O 4 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 Fe 3 O 4 and γ-Fe 2 O 3 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-Fe 2 O 3 shell with magnetic properties distinct from the Fe 3 O 4 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-Fe 2 O 3 , 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. This article is protected by copyright. All rights reserved.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Does size matter? Statistical limits of paleomagnetic field reconstruction from small rock specimens (2015)
    Wiley
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    Publication Date: 2015-12-16
    Description: As samples of ever decreasing sizes are being studied paleomagnetically, care has to be taken that the underlying assumptions of statistical thermodynamics (Maxwell-Boltzmann statistics) are being met. Here, we determine how many grains and how large a magnetic moment a sample needs to have to be able to accurately record an ambient field. It is found that for samples with a thermoremanent magnetic moment larger than 10 −11 Am 2 the assumption of a sufficiently large number of grains is usually given. Standard 1 inch diameter paleomagnetic samples usually contain enough magnetic grains such that statistical errors are negligible, but ‘single silicate crystal’ works on, for example, zircon, plagioclase and olivine crystals are approaching the limits of what is physically possible, leading to statistic errors in both the angular deviation and paleointensity that are comparable to other sources of error. The reliability of nano-paleomagnetic imaging techniques capable of resolving individual grains (used, for example, to study the cloudy zone in meteorites), however, is questionable due to the limited area of the material covered.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 4
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    Paleointensity determinations from the Etendeka province, Namibia, support a low magnetic field strength leading up to the Cretaceous normal superchron (2015)
    Wiley
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    Publication Date: 2015-02-20
    Description: Paleointensity estimates provide much needed information on field generation within Earth's core and upon the convective processes at work within the mantle. We present new paleointensity estimates from the early Cretaceous Etendeka large igneous province in Namibia (~135Ma) which add to the sparse southern hemisphere dataset. The Early Cretaceous marks an important change in the Earth's magnetic field from a state of rapid polarity reversals, to one of long-term stability associated with the onset of the Cretaceous Normal Superchron at c. 121Ma. Paleointensity determinations, using the IZZI protocol, were carried out on a total of 172 specimens from 14 sites encompassing the exposed stratigraphy of the Etendeka province. Numerous checks of data reliability were considered before results were accepted, including partial thermoremanent magnetisation (pTRM) checks and pTRM tail checks, hysteresis properties, thermomagnetic analyses, observations under reflected light, and changes to room-temperature susceptibility during the experiments. Following these checks a total of 64 individual samples from 5 sites were considered to provide reliable paleointensity determinations. These results were combined to provide site mean data with an overall average virtual dipole moment (VDM) for the study of 2.5 ± 1.0 x 10 22 Am 2 . This value equates to approximately 30% of present Earth's field and, when considered alongside existing studies, suggests that Earth's field strength was low in the time leading up to the Cretaceous normal superchron. This article is protected by copyright. All rights reserved.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 5
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    Determining the magnetic attempt time τ0, its temperature dependence and the grain-size distribution from magnetic viscosity measurements (2015)
    Wiley
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    Publication Date: 2015-10-30
    Description: A new method to determine the atomic attempt time τ 0 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.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 6
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    Domain State Diagnosis in Rock Magnetism: Evaluation of Potential Alternatives to the Day Diagram (2019)
    Wiley
    Details
    Publication Date: 2019
    Description: Abstract The Day diagram is used extensively in rock magnetism for domain state diagnosis. It has been shown recently to be fundamentally ambiguous for 10 sets of reasons. This ambiguity highlights the urgency for adopting suitable alternative approaches to identify the domain state of magnetic mineral components in rock magnetic studies. We evaluate 10 potential alternative approaches here and conclude that four have value for identifying data trends, but, like the Day diagram, they are affected by use of bulk parameters that compromise domain state diagnosis in complex samples. Three approaches based on remanence curve and hysteresis loop unmixing, when supervised by independent data to avoid nonuniqueness of solutions, provide valuable component‐specific information that can be linked by inference to domain state. Three further approaches based on first‐order reversal curve diagrams provide direct domain state diagnosis with varying effectiveness. Environmentally important high‐coercivity hematite and goethite are represented with variable effectiveness in the evaluated candidate approaches. These minerals occur predominantly in noninteracting single‐domain particle assemblages in paleomagnetic contexts, so domain state diagnosis is more critical for ferrimagnetic minerals. Treating the high‐coercivity component separately following normal rock magnetic procedures allows focus on the more vexing problem of diagnosing domain state in ferrimagnetic mineral assemblages. We suggest a move away from nondiagnostic methods based on bulk parameters and adoption of approaches that provide unambiguous component‐specific domain state identification, among which various first‐order reversal curve‐based approaches provide diagnostic information.
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
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  • 7
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    From Nano to Micro: Evolution of Magnetic Domain Structures in Multidomain Magnetite (2019)
    Wiley
    Details
    Publication Date: 2019
    Description: Abstract Reliability 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 ( 10 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.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 8
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    A Preisach method for estimating absolute paleofield intensity under the constraint of using only isothermal measurements: 2. Experimental testing (2011)
    Wiley
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    Publication Date: 2011-04-08
    Description: This paper reports an empirical test of a new nonheating Preisach-based protocol for determining the absolute ancient magnetic field intensity (paleointensity) using a selection of synthetic samples and a large suite of modern lavas and pyroclastic lithic samples. Generally, the Preisach paleointensity estimates compare favorably with the expected field intensities: (1) for the synthetic samples displaying single-domain-like behavior, the method returned the correct result for the known field, while multidomain samples yielded an underestimate, and (2) averaging 168 post-1850 A.D. lavas yielded a value
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 9
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    A Preisach method for estimating absolute paleofield intensity under the constraint of using only isothermal measurements: 1. Theoretical framework (2011)
    Wiley
    Details
    Publication Date: 2011-04-08
    Description: The theoretical framework for a new nonheating method of determining absolute ancient magnetic field intensities (paleointensities) is described. The approach is based on a thermally activated Preisach model for interacting, randomly orientated single-domain grains with uniaxial anisotropy. The model includes theoretical features not accommodated by previous nonheating paleointensity methods; for example, it includes magnetostatic interactions, allows for variable cooling rates, and can identify, isolate, and reject unstable remanence carriers, i.e., multidomain and superparamagnetic contributions. The input Preisach distribution from which the acquisition of a thermal remanent magnetization (TRM) of a given rock sample can be simulated is obtained from information contained in the sample's first-order reversal curve distribution. The paleointensity estimate is determined by comparing the alternating field demagnetization spectrum of the sample's natural remanent magnetization and its simulated TRM. In the companion paper, the protocol is rigorously tested using a suite of historical samples.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 10
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    Magnetic characteristics of CV chondrules with paleointensity implications (2011)
    Wiley
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    Publication Date: 2011-12-29
    Description: We have conducted a detailed magnetic study on 45 chondrules from two carbonaceous chondrites of the CV type: (1) Mokoia and (2) Allende. Allende has been previously extensively studied and is thought to have a high potential of retaining an extra-terrestrial paleofield. Few paleomagnetic studies of Mokoia have previously been undertaken. We report a range of magnetic measurements including hysteresis, first-order reversal curve analysis (FORCs), demagnetization characteristics, and isothermal remanent (IRM) acquisition behavior on both Mokoia and Allende chondrules. The Mokoia chondrules displayed more single domain-like behavior than the Allende chondrules, suggesting smaller grain sizes and higher magnetic stability. The Mokoia chondrules also had higher average concentrations of magnetic minerals and a larger range of magnetic characteristics than the Allende chondrules. IRM acquisition analysis found that both sets of chondrules have the same dominant magnetic mineral, likely to be a FeNi phase (taenite, kamacite, and/or awaruite) contributing to 48% of the Mokoia chondrules and 42% of the Allende chondrule characteristics. FORC analysis revealed that generally the Allende chondrules displayed low-field coercivity distributions with little interactions, and the Mokoia chondrules show clear single-domain like distributions. Paleointensity estimates for the two meteorites using the REMc and Preisach methods yielded estimates between 13 and 60 μT and 3–56 μT, respectively, for Allende and 3–140 μT and 1–110 μT, respectively, for Mokoia. From the data, we suggest that Mokoia chondrules carry a non-primary remagnetization, and while Allende is more likely than Mokoia to retain its primary magnetization, it also displays signs of post accretionary magnetization.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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