Publications
339 results found
Muxworthy AR, Williams W, 2005, Low-temperature viscous magnetization of multidomain magnetite: evidence for disaccommodation contribution (poster)
Williams W, Muxworthy AR, Paterson G, 2005, Configurational anisotropy in equidimensional grains (poster), AGU Fall
Carvallo C, Muxworthy AR, Dunlop DJ, 2005, Effect of magnetostatic interactions on the linear additivity of FORC diagrams, EGU
Muxworthy AR, Williams W, 2005, Low-temperature cooling behavior of single-domain magnetite: forcing of the crystallographic axes and interactions (invited), AGU Fall
Muxworthy AR, Williams W, 2005, Domain observations of multidomain magnetite: sub-micron structures, dislocations and viscosity, EGU
Muxworthy A, Heslop D, Williams W, 2004, Influence of magnetostatic interactions on first-order-reversal-curve (FORC) diagrams: a micromagnetic approach, GEOPHYSICAL JOURNAL INTERNATIONAL, Vol: 158, Pages: 888-897, ISSN: 0956-540X
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- Citations: 105
Muxworthy A R, Williams W, 2004, Distribution anisotropy: the influence of magnetic interactions on the anisotropy of magnetic remanence, Magnetic Fabric: Methods and Applications, Editors: Martín-Hernández F, Lüneburg M, Aubourg C, Jackson M, London, Publisher: Geological Society, Pages: 37-47, ISBN: 1-86239-170-X
Williams W, Muxworthy AR, Ridley P, 2004, Magnetite morphology and micromagnetic modelling: Characteristics of magnetotactic bacteria (invited), AGU Fall
Williams W, Muxworthy AR, 2004, Magnetostatic interaction fields in First-order-reversal-curve (FORC) diagrams (poster)
Muxworthy AR, Williams W, 2004, Viscous behaviour of multidomain magnetite, AGU Fall
Muxworthy A, Williams W, Virdee D, 2003, Effect of magnetostatic interactions on the hysteresis parameters of single-domain and pseudo-single-domain grains, Journal of Geophysical Research: Solid Earth, Vol: 108, ISSN: 2169-9313
From experiments it is known that magnetostatic interactions between grains strongly affect the magnetic behavior of samples. However, because of the difficulty in predicting the nonlinear behavior, the effect of interactions has been largely ignored from theoretical models. Instead models are often based on noninteracting assemblages. This approximation is valid for certain natural systems, but there are many cases where interactions are known to be important, for example, bacterial magnetosomes found in sedimentary rocks. Using a three-dimensional micromagnetic model, we have conducted a detailed study of the role of magnetostatic interactions on the magnetic properties of assemblages of ideal single domain (SD) grains and cubic grains between 30-250 nm in size. We quantify the contribution of interactions to hysteresis parameters and the Day plot. We show that interactions can strongly affect the magnetic characteristics of a grain assemblage. For example, assemblages of interacting SD grains can plot in the traditional multidomain (MD) area of the Day plot. For grains >100 nm in size, interactions can have the opposite effect, and can cause the hysteresis parameters to shift toward the SD region of the Day plot. In addition to varying grain size, we have also considered various anisotropies, e.g., uniaxial and cubic, and the importance of the alignment configuration of the particle assemblages, i.e., randomly distributed or aligned. It is shown that for assemblages of aligned magnetite particles, that as the interaction spacing is decreased, the SD/MD transition size increases, which may explain why some magnetotatic, bacteria possess aligned grains of magnetite above the traditional transition size value of 70 nm. By aligning the anisotropies, the grains become stable SD, and having larger crystals will increase the magnetic signal. Copyright 2003 by the American Geophysical Union.
Muxworthy A, Williams W, Virdee D, 2003, Effect of magnetostatic interactions on the hysteresis parameters of single-domain and pseudo-single-domain grains, JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, Vol: 108, ISSN: 2169-9313
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- Citations: 61
Muxworthy AR, Matzka M, Davila AF, et al., 2003, Magnetic signature of daily sampled urban atmospheric particles, ATMOSPHERIC ENVIRONMENT, Vol: 37, Pages: 4163-4169, ISSN: 1352-2310
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- Citations: 93
Carvallo C, Muxworthy AR, Dunlop DJ, et al., 2003, Micromagnetic modeling of first-order reversal curve (FORC) diagrams for single-domain and pseudo-single-domain magnetite, EARTH AND PLANETARY SCIENCE LETTERS, Vol: 213, Pages: 375-390, ISSN: 0012-821X
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- Citations: 47
Muxworthy AR, Dunlop DJ, Williams W, 2003, High-temperature magnetic stability of small magnetite particles, JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, Vol: 108, ISSN: 2169-9313
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- Citations: 41
Muxworthy AR, Dunlop DJ, Williams W, 2003, High temperature magnetic stability of small magnetic particles, Journal of Geophysical Research: Solid Earth, Vol: 108, ISSN: 2169-9313
The stability of magnetic domain structures of small grains of magnetite were examined between room temperature and the Curie temperature using a high-resolution three-dimensional micromagnetic algorithm. At all times the minimum resolution used was determined by calculating the exchange length. Using an unconstrained model, the single domain (SD) to multidomain (MD) threshold grain size do was found to be nearly independent of temperature up to ∼450°C. Above this temperature, do was observed to rise sharply. Energy barriers between metastable domain states trapped in local energy minimums (LEM) were determined using a constrained algorithm. Three types of domain structure were considered: SD, vortex, and double vortex (effectively three domain), in a range of grain sizes with side length between 30 and 300 nm. In addition, the effect of varying shape was also considered by examining asymmetric grains with aspect ratios up to 1.4. From the numerical solutions energy barriers between LEM states were determined. It was found that MD grains 300 nm in size display higher stability than smaller SD grains (∼50 nm). Double vortex states were found to be less stable than single vortex states at nearly all temperatures. Blocking temperatures as function of grain size for both symmetric and asymmetric grains were determined and agree well with experimental results. Transdomain thermoremanence analysis indicated that there are a limited number of grain sizes and shapes which will nucleate domain wall-type structures during cooling. Such nucleation events would cause the total measured remanence to decrease with cooling in conflict with Néel's analytical theory for remanence cooling behavior but in agreement with experimental observations.
Broadhead JS, Muxworthy AR, Ong CK, et al., 2003, Comparison of methods for determining leaf area in tree rows, AGRICULTURAL AND FOREST METEOROLOGY, Vol: 115, Pages: 151-161, ISSN: 0168-1923
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- Citations: 16
Muxworthy AR, Dunlop DJ, Özdemir Ö, 2003, Low-temperature cycling of isothermal and anhysteretic remanence:: microcoercivity and magnetic memory, EARTH AND PLANETARY SCIENCE LETTERS, Vol: 205, Pages: 173-184, ISSN: 0012-821X
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- Citations: 34
Muxworthy AR, King JG, Heslop D, et al., 2003, Unravelling Magnetic Mixtures Using First-Order-Reversal-Curve (FORC) Diagrams: Linear Additivity and Interactions (poster), AGU Fall
Muxworthy AR, Williams W, Virdee D, et al., 2003, Micromagnetic modelling of magnetostatic interactions (invited), EGS-AGU-EUG
Muxworthy AR, McClelland E, Williams W, 2003, Understanding viscous magnetisation in multidomain minerals, IUGG
Muxworthy AR, Williams W, Virdee D, et al., 2003, Micromagnetic modelling of magnetostatic interactions: hysteresis and FORC diagrams (invited), 1st International Workshop on Magnetism, Hysteresis and the FORC method
Williams W, Ridley P, Muxworthy AR, 2003, The Effect of Grain Shape on the Magnetic Properties of Magnetite: A Finite Element Approach, AGU Fall
Muxworthy AR, Williams W, 2003, The Role of Magnetostatic Interactions on the Anisotropy of Magnetic Remanence: Micromagnetic Modeling of Distribution Anisotropy (invited), AGU Fall
Carvallo C, Dunlop DJ, Ozdemir O, et al., 2003, Testing some predicted properties of FORC diagrams: Micromagnetic modeling and measurements (invited), 1st International Workshop on Magnetism, Hysteresis and the FORC method
Muxworthy AR, Dunlop DJ, Ozdemir O, 2003, Low-temperature cycling of isothermal and anhysteretic remanence: microcoercivity and magnetic memory, IUGG
Muxworthy A R, Carvallo C, Dunlop D J, 2002, Micromagnetic modeling of First-Order Reversal Curves (FORC) diagrams for single-domain pseudo-single-domain magnetite (poster), AGU Fall
Muxworthy AR, Dunlop DJ, 2002, First-order reversal curve (FORC) diagrams for pseudo-single-domain magnetites at high temperature, EARTH PLANET SC LETT, Vol: 203, Pages: 369-382, ISSN: 0012-821X
The recently developed first-order reversal curve (FORC) technique for rapidly examining magnetic domain state has great potential for paleomagnetic and environmental magnetic investigations. However, there are still some gaps in the basic understanding of FORC diagrams, in particular the behavior of pseudo-single-domain (PSD) grains and the contribution of magnetostatic interactions. In this paper we address some of these problems. We report the first FORC diagrams measurements on narrowly sized and well-characterized synthetic PSD through multidomain (MD) magnetite samples. The FORC diagrams evolve with grain size from single-domain (SD)-like to MD-like through the PSD grain size range. Since each sample contains grains of essentially a single size, individual PSD grains evidently contain contributions from both SD-like and MD-like magnetic moments, in proportions that vary with grain size; the evolving FORC diagrams cannot be due to physical mixtures of SD and MD grains of widely different sizes. The FORC diagrams were all asymmetric. Small PSD samples have FORC diagrams with a distinctive closed-contour structure. The distributions of the larger MD grains display no peak, and lie closer to the interaction-field axis. To assess the effect of magnetostatic interactions, we measured FORC diagrams between room temperature and the Curie temperature. On heating the FORC distributions contract without changing shape until similar to500degreesC. Above this temperature the diagrams become more MD-like, and in addition become more symmetric. The temperature dependence of the interaction-field parameter is proportional to that of the saturation magnetization, in accordance with Neel's interpretation of the Preisach diagram. The decrease in asymmetry with heating suggests that the origin of the asymmetry lies in magnetostatic interactions. The magnetic hysteresis parameters as a function of temperature were determined from the FORC curves. As the grain size decreased the no
Muxworthy AR, Schmidbauer E, Petersen N, 2002, Magnetic properties and Mossbauer spectra of urban atmospheric particulate matter: a case study from Munich, Germany, GEOPHYSICAL JOURNAL INTERNATIONAL, Vol: 150, Pages: 558-570, ISSN: 0956-540X
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- Citations: 102
Muxworthy AR, 2002, Magnetic hysteresis and rotational hysteresis properties of hydrothermally grown multidomain magnetite, GEOPHYSICAL JOURNAL INTERNATIONAL, Vol: 149, Pages: 805-814, ISSN: 0956-540X
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- Citations: 14
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