ORCID Profile
0000-0002-4392-7784
Current Organisation
The University of Newcastle
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Publisher: CSIRO Publishing
Date: 2014
DOI: 10.1071/CH14136
Abstract: Three thiacalix[4]arene polynuclear complexes have been prepared by literature methods for detailed magnetic investigation. The [Fe3O(L)2] (LH4 = thiacalix[4]arene) complex is found to exhibit interesting anti-ferromagnetic exchange coupling. Jahn–Teller distortion in [Cu4(L)2] complex leads to strong anti-ferromagnetic coupling at low temperatures. The temperature-dependent susceptibility of the [(μ-H2O)Eu2(LH)2(DMF)4] complex is well described by a ground state involving the thermal population of the lowest three excited states.
Publisher: American Physical Society (APS)
Date: 02-08-2019
Publisher: Elsevier
Date: 2015
Publisher: American Physical Society (APS)
Date: 11-10-2023
Publisher: American Physical Society (APS)
Date: 27-06-2011
Publisher: CSIRO Publishing
Date: 07-07-2021
DOI: 10.1071/CH21061
Abstract: The translation of nanoparticles to useful applications is often hindered by the reliability of synthetic methodologies to reproducibly generate larger particles of uniform size (diameter 20 nm). The inability to precisely control nanoparticle crystallinity, size, and shape has significant implications on observed properties and therefore applications. A series of iron oxide particles have been synthesised and the impact of size as they agglomerate in aqueous media undergoing flow through a capillary tube has been studied. Reaction conditions for the production of large (side length 40 nm) cubic magnetite (Fe3O4) have been optimised to produce particles with different diameters up to 150 nm. We have focussed on reproducibility in synthesis rather than dispersity of the size distribution. A simple oxidative cleavage of the as-synthesised particles surfactant coating transforms the hydrophobic oleic acid coated Fe3O4 to a hydrophilic system based on azelaic acid. The hydrophilic coating can be further functionalised, in this case we have used a simple biocompatible polyethylene glycol (PEG) coating. The ability of particles to either chain, flow, and fully/or partially aggregate in aqueous media has been tested in a simple in-house system made from commercial components. Fe3O4 nanoparticles (60–85 nm) with a simple PEG coating were found to freely flow at a 2 mm distance from a magnet over 3 min at a rate of 1 mL min−1. Larger particles with side lengths of ~150 nm, or those without a PEG coating were not able to fully block the tube. Simple calculations have been performed to support these observations of magnetic agglomeration.
Publisher: Elsevier BV
Date: 10-2023
Publisher: American Physical Society (APS)
Date: 18-05-2007
Publisher: Springer Science and Business Media LLC
Date: 24-07-2018
DOI: 10.1038/S41598-018-29501-8
Abstract: We consider the probability of a magnetic nanoparticle to flip its magnetisation near the blocking temperature, and use this to develop quasi-analytic expressions for the zero-field-cooled and field-cooled magnetisation, which go beyond the usual critical energy barrier approach to the superparamagnetic transition. The particles in the assembly are assumed to have random alignment of easy axes, and to not interact. We consider all particles to be of the same size and then extend the theory to treat polydisperse systems of particles. In particular, we find that the mode blocking temperature is at a lower temperature than the peak in the zero-field-cooled magnetisation versus temperature curve, in agreement with experiment and previous rate-equation simulations, but in contrast to the assumption many researchers use to analyse experimental data. We show that the quasi-analytic expressions agree with Monte Carlo simulation results but have the advantage of being very quick to use to fit data. We also give an ex le of fitting experimental data and extracting the anisotropy energy density K .
Publisher: American Physical Society (APS)
Date: 14-11-2011
Publisher: AIP Publishing
Date: 28-01-2018
DOI: 10.1063/1.5003660
Abstract: The parametric excitation of spin waves in a rectangular, ferromagnetic nanowire in the parallel pump configuration and with an applied field along the long axis of the wire is studied theoretically, using a semi-classical and semi-analytic Hamiltonian approach. We find that as a function of static applied field strength, there are jumps in the pump power needed to excite thermal spin waves. At these jumps, there is the possibility to non-resonantly excite spin waves near kz = 0. Spin waves with negative or positive group velocity and with different standing wave structures across the wire width can be excited by tuning the applied field. By using a magnetostatic Green's function that depends on both the nanowire's width and thickness—rather than just its aspect ratio—we also find that the threshold field strength varies considerably for nanowires with the same aspect ratio but of different sizes. Comparisons between different methods of calculations are made and the advantages and disadvantages of each are discussed.
Publisher: Elsevier BV
Date: 05-2004
Publisher: American Physical Society (APS)
Date: 25-05-2006
Publisher: AIP Publishing
Date: 05-08-2013
DOI: 10.1063/1.4818466
Abstract: The phase-shift per unit-length of microwave radiation passing through a liquid crystal cell is calculated. By using twisted nematics, as compared to the standard homogeneous nematic liquid crystal, the phase-shift per unit-length is increased over the transition region. This has the potential to lower the operating voltage and decrease the size of such phase shifters. In particular, the phase shifts for microwave radiation produced by the liquid crystals K15, E7, and E44 are calculated. It is found that liquid crystals with large values of the elastic constant ratio K22/K11 have their performance most improved when a twist is introduced.
Publisher: Elsevier BV
Date: 2019
Publisher: AIP Publishing
Date: 21-06-2010
DOI: 10.1063/1.3457478
Abstract: We calculate the attenuation in microwave waveguides comprising a thin film of dielectric SiO2 sandwiched by copper or iron films. We show that in a frequency range between roughly 63 and 73 GHz, using iron gives lower losses by up to 7.5 dB/cm compared with copper. This is in the region of so-called antiresonance where the effective skin depth of ferromagnetic iron erges.
Publisher: American Physical Society (APS)
Date: 04-02-2010
Publisher: American Physical Society (APS)
Date: 24-07-2020
Publisher: American Physical Society (APS)
Date: 17-02-2017
Publisher: AIP Publishing
Date: 05-2011
DOI: 10.1063/1.3573497
Abstract: We explore the response of a magnetic bilayer to a driving microwave field using micromagnetic simulations. The bilayer consists of 8 nm of a material with a high uniaxial anisotropy and 56 nm of a material with a lower uniaxial anisotropy. The width and length of the structure is 100 × 100 square microns. A small applied field, opposite to the magnetization, switches most of the lower anisotropy material but not the higher anisotropy material, forming a domain wall between the two materials. We evaluate the frequencies of the magnetic eigenmodes for the entire system using Fourier analysis and then drive the structure with an oscillating magnetic field at each of the eigenfrequencies. When the oscillating microwave field is added, the static switching field required to align both layers is decreased compared to the undriven case. With a driving field strength of 120 Oe the switching field is reduced by about 40%, from 1.12 kOe for the undriven case to 0.55 Oe for the driven case.
Publisher: Elsevier BV
Date: 02-2022
Publisher: American Physical Society (APS)
Date: 25-02-2021
Publisher: American Physical Society (APS)
Date: 14-12-2015
Publisher: Elsevier BV
Date: 11-2020
Publisher: Elsevier BV
Date: 12-2021
Publisher: American Association of Physics Teachers (AAPT)
Date: 06-2019
DOI: 10.1119/1.5102146
Abstract: A finite difference method is used to determine the effective or average properties of a two-dimensional composite material. The host material and several inclusions dispersed through it have different values for the electric permittivity, and the problem is to compute the permittivity of the total structure. Results are explored as a function of the filling factor of the inclusions and of the mismatch between the permittivity values of the two constituent materials. The results are compared to the predictions of analytic, approximate effective medium methods. The calculations can be extended to treat a variety of electrostatic and magnetostatic problems.
Publisher: Elsevier BV
Date: 12-2014
Publisher: Informa UK Limited
Date: 16-01-2014
Publisher: American Physical Society (APS)
Date: 09-08-2010
Publisher: American Physical Society (APS)
Date: 25-04-2022
Publisher: Wiley
Date: 10-2020
Publisher: MDPI AG
Date: 27-10-2021
DOI: 10.3390/NANO11112870
Abstract: The agglomeration of ferromagnetic nanoparticles in a fluid is studied using nanoparticle-level Langevin dynamics simulations. The simulations have interdigitation and bridging between ligand coatings included using a computationally-cheap, phenomenological sticking parameter c. The interactions between ligand coatings are shown in this preliminary study to be important in determining the shapes of agglomerates that form. A critical size for the sticking parameter is estimated analytically and via the simulations and indicates where particle agglomerates transition from well-ordered (c is small) to disordered (c is large) shapes. Results are also presented for the hysteresis loops (magnetization versus applied field) for these particle systems in an oscillating magnetic field appropriate for hyperthermia applications. The results show that the clumping of particles has a significant effect on their macroscopic properties, with important consequences on applications. In particular, the work done by an oscillating field on the system has a nonmonotonic dependence on c.
Publisher: American Physical Society (APS)
Date: 28-09-2021
Publisher: AIP Publishing
Date: 15-03-2017
DOI: 10.1063/1.4978702
Abstract: Electromagnetic shielding at microwave frequencies (MHz and GHz) can be accomplished by attenuating the waves using ferromagnetic resonance and eddy currents in conductive materials. This method is not as effective at shielding the quasi-static magnetic fields produced by low-frequency (kHz) currents. We explore theoretically the use of composite materials – magnetic nanoparticles embedded in a polymer matrix – as a shielding material surrounding a 3-phase current source. We develop several methods to estimate the permeability of a single magnetic nanoparticle at low frequencies, several hundred kHz, and find that the relative permeability can be as high as 5,000-20,000. We then use two analytic effective medium theories to find the effective permeability of a collection of nanoparticles as a function of the volume filling fraction. The analytic calculations provide upper and lower bounds on the composite permeability, and we use a numerical solution to calculate the effective permeability for specific cases. The field-pattern for the 3-phase current is calculated using a magnetic scalar potential for each of the three wires surrounded by a cylinder with the effective permeability found above. For a cylinder with an inner radius of 1 cm and an outer radius of 1.5 cm and an effective permeability of 50, one finds a reduction factor of about 8 in the field strength outside the cylinder.
Publisher: AIP Publishing
Date: 03-08-2016
DOI: 10.1063/1.4959987
Abstract: The effective microwave permittivity of a composite comprising anisotropic particles suspended in a liquid (also known as an artificial Kerr material) is calculated using a numerical scheme. The results are compared to those calculated using analytic effective medium expressions. Several composite materials are predicted to have permittivity that can be tuned over a greater range than liquid crystals, which are currently used in many tunable microwave devices such as phase-shifters and modulators. Estimates for the static electric field required to tune such materials are provided and show that such materials are realistic.
Publisher: Wiley
Date: 17-08-2022
Abstract: Colloidal magnetite nanoparticles self‐assemble onto a disk drive medium as directed by magnetic field gradients created where the medium magnetic moment switches direction over single nanometer distances. Here, it is shown that for two such reversals or transitions that are closely spaced, the nanoparticles self‐assemble into a single feature centered between the transitions, rather than forming separate features at the transitions, and the resulting 2D assembly achieves hexatic ordering. Langevin dynamics simulations are used to explain these results, and it is found that the detailed magnetic properties of the medium play a critical role in determining assembly location. Slight changes to solvent polarity disrupt the hexatic ordering and push the nanoparticles toward the transitions, suggesting an alternate mechanism to precisely tune the self‐assembly process.
Publisher: American Physical Society (APS)
Date: 28-02-2013
Publisher: Springer Science and Business Media LLC
Date: 09-2003
DOI: 10.1007/S00239-003-2492-8
Abstract: Carboxylesterases hydrolyze esters of short-chain fatty acids and have roles in animals ranging from signal transduction to xenobiotic detoxification. In plants, however, little is known of their roles. We have systematically mined the genome from the model plant Arabidopsis thaliana for carboxylesterase genes and studied their distribution in the genome and expression profile across a range of tissues. Twenty carboxylesterase genes (AtCXE) were identified. The AtCXE family shares conserved sequence motifs and secondary structure characteristics with carboxylesterases and other members of the larger alpha/beta hydrolase fold superfamily of enzymes. Phylogenetic analysis of the AtCXE genes together with other plant carboxylesterases distinguishes seven distinct clades, with an Arabidopsis thaliana gene represented in six of the seven clades. The AtCXE genes are widely distributed across the genome (present in four of five chromosomes), with the exception of three clusters of tandemly duplicated genes. Of the interchromosomal duplication events, two have been mediated through newly identified partial chromosomal duplication events that also include other genes surrounding the AtCXE loci. Eighteen of the 20 AtCXE genes are expressed over a broad range of tissues, while the remaining 2 (unrelated) genes are expressed only in the flowers and siliques. Finally, hypotheses for the functional roles of the AtCXE family members are presented based on the phylogenetic relationships with other plant carboxylesterases of known function, their expression profile, and knowledge of likely esterase substrates found in plants.
Publisher: AIP Publishing
Date: 14-11-2022
DOI: 10.1063/5.0124550
Abstract: We have explored dynamic magnetic properties of micron-sized Ni-coated carbon fibers embedded in a polymer matrix for electromagnetic interference shielding applications. These hollow magnetic cylinders exhibit unusual dynamic magnetic properties, which were measured with a broad-band ferromagnetic resonance system (FMR). We observe three families of FMR modes, which are connected to different physical locations within the cylinder. We develop a simple analytic model to explain these results and corroborate resonant mode profiles with micromagnetic simulations. We find excellent agreement between experimental results and theoretical models. Our work indicates that global demagnetizing factors are not appropriate for understanding the spin motions in these hollow cylinders. The FMR absorption observed in these hallow cylinders is very different from those observed in nanowires or solid cylinders. The field-swept envelope of all the observed FMR resonances is very broad, approximately μ0H = 1 T, with a linewidth of in idual modes around μ0ΔH = 250 mT. This can be important for electromagnetic shielding applications.
Publisher: American Physical Society (APS)
Date: 10-12-2020
Publisher: American Chemical Society (ACS)
Date: 26-01-2007
DOI: 10.1021/BI062046W
Abstract: Carboxylesterases (CXEs) are widely distributed in plants, where they have been implicated in roles that include plant defense, plant development, and secondary metabolism. We have cloned, overexpressed, purified, and crystallized a carboxylesterase from the kiwifruit species Actinidia eriantha (AeCXE1). The structure of AeCXE1 was determined by X-ray crystallography at 1.4 A resolution. The crystal structure revealed that AeCXE1 is a member of the alpha/beta-hydrolase fold superfamily, most closely related structurally to the hormone-sensitive lipase subgroup. The active site of the enzyme, located in an 11 A deep hydrophobic gorge, contains the conserved catalytic triad residues Ser169, Asp276, and His306. Kinetic analysis using artificial ester substrates showed that the enzyme can hydrolyze a range of carboxylester substrates with acyl groups ranging from C2 to C16, with a preference for butyryl moieties. This preference was supported by the discovery of a three-carbon acyl adduct bound to the active site Ser169 in the native structure. AeCXE1 was also found to be inhibited by organophosphates, with paraoxon (IC50 = 1.1 muM) a more potent inhibitor than dimethylchlorophosphate (DMCP IC50 = 9.2 muM). The structure of AeCXE1 with paraoxon bound was determined at 2.3 A resolution and revealed that the inhibitor binds covalently to the catalytic serine residue, with virtually no change in the structure of the enzyme. The structural information for AeCXE1 provides a basis for addressing the wider functional roles of carboxylesterases in plants.
Publisher: Wiley
Date: 31-05-2006
DOI: 10.1111/J.1365-2583.2006.00656.X
Abstract: RNA interference (RNAi) or gene silencing is typically induced in insects by the injection of double-stranded RNAs (dsRNAs), short interfering RNAs, or through the use of hairpin constructs in transgenic insects. Here we demonstrate in the horticultural pest, Epiphyas postvittana (Lepidoptera: Tortricidae), that RNAi can be triggered by oral delivery of dsRNA to larvae. Transcript levels of a larval gut carboxylesterase gene (EposCXE1) were reduced to less than half that of controls within 2 days of being fed EposCXE1 dsRNA. Transcript levels of the pheromone binding protein gene (EposPBP1) were reduced in adult antennae by feeding larvae EposPBP1 dsRNA. Knockdown of EposPBP1 transcripts was observed for the first 2 days after adult eclosion but recovered to wild-type levels at 4 days posteclosion. The potential mechanisms involved in the initiation, movement and lification of the silencing signal are discussed.
Publisher: American Physical Society (APS)
Date: 05-03-2010
Publisher: IOP Publishing
Date: 20-03-2012
Publisher: IOP Publishing
Date: 02-2015
Publisher: American Physical Society (APS)
Date: 29-01-2020
Publisher: AIP Publishing
Date: 17-09-2018
DOI: 10.1063/1.5049602
Abstract: Metamaterials have enabled a series of major advances in optical devices in the past decade. Here, we suggest a type of hyperbolic metamaterial based on spin canting in magnetic multi-layers. We show that these structures have unique features in microwave waveguides that act as tunable filters. In the resulting band pass filter, we demonstrate an exceptional frequency tunability of 30 GHz with external fields smaller than 500 Oe. Unlike single metallic ferromagnetic films, we also demonstrate a high-frequency band-stop filter at very low fields.
Publisher: Elsevier BV
Date: 06-2014
DOI: 10.1016/J.JCIS.2014.03.007
Abstract: The design and application of magnetic nanoparticles for use as magnetic hyperthermia agents has garnered increasing interest over the past several years. When designing these systems, the fundamentals of particle design play a key role in the observed specific absorption rate (SAR). This includes the particle's core size, polymer brush length, and colloidal arrangement. While the role of particle core size on the observed SAR has been significantly reported, the role of the polymer brush length has not attracted as much attention. It has recently been reported that for some suspensions linear aggregates form in the presence of an applied external magnetic field, i.e. chains of magnetic particles. The formation of these chains may have the potential for a dramatic impact on the biomedical application of these materials, specifically the efficiency of the particles to transfer magnetic energy to the surrounding cells. In this study we demonstrate the dependence of SAR on magnetite nanoparticle core size and brush length as well as observe the formation of magnetically induced colloidal arrangements. Colloidally stable magnetic nanoparticles were demonstrated to form linear aggregates in an alternating magnetic field. The length and distribution of the aggregates were dependent upon the stabilizing polymer molecular weight. As the molecular weight of the stabilizing layer increased, the magnetic interparticle interactions decreased therefore limiting chain formation. In addition, theoretical calculations demonstrated that interparticle spacing has a significant impact on the magnetic behavior of these materials. This work has several implications for the design of nanoparticle and magnetic hyperthermia systems, while improving understanding of how colloidal arrangement affects SAR.
Publisher: Wiley
Date: 12-09-2021
Abstract: The consequences of non‐uniform exchange in magnetic systems are reported. The quantum mechanical exchange interaction between spins is responsible for the phenomenon of magnetic order, and is generally considered to be uniform across bulk magnetic systems. Partly inspired by the Dzyaloshinskii‐Moriya interaction—also known as antisymmetric exchange—a linearly varying exchange interaction is used along a magnetic strip as a route to spatial inversion symmetry‐breaking. It is found that, in addition to asymmetric modes and localization, spatially varying exchange can be used to design nonreciprocal magnetic signal excitation at frequencies that are tunable. Moreover, the authors’ work predicts nonreciprocity to occur across a vast range of frequencies up to hundreds of GHz. Such spin wave engineering is a key area of ongoing research in the fields of magnonics and spintronics, which are expected to enable the next generation of wireless communication technology and information processing. Analogous nonreciprocity is expected to occur in other wave systems with gradient properties.
Publisher: American Physical Society (APS)
Date: 24-06-2020
Publisher: AIP Publishing
Date: 27-02-2023
DOI: 10.1063/5.0135956
Abstract: Electromagnetic interference shielding composite materials can be designed with properties that are tailored for specific applications. We have explored the magnetic properties of Ni-coated carbon fibers embedded in a polymer matrix through an extrusion process. This method can produce large, pliable flat sheets, which can be easily bent and shaped. Microscopy imaging shows that the extrusion process preferentially orders most of the fibers with their long axis along the extrusion direction. The measured s les reached 90% of the saturation magnetization value at a field of approximately 800 Oe along the extrusion direction, much lower than needed for other directions (in-plane perpendicular to the extrusion or perpendicular to the foil surface). A field on the order of 6 kOe must be applied to obtain the full saturation of the magnetization in all three directions. The absorption characteristics were measured, with zero external applied field to determine the shielding effectiveness of the composite materials for multiple orientations with respect to the TE10 mode of an S-band waveguide. The nickel-coated carbon fiber composite materials exhibit dramatic orientation-dependent shielding effectiveness. The most effective orientation has shielding effectiveness of up to 45 dB at 4.5 GHz, which is comparable to multi-walled carbon nanotube composites with similar volumetric filling fractions.
Location: United States of America
Start Date: 2018
End Date: 2021
Funder: Directorate for Mathematical & Physical Sciences
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