Steve R. Hoon

Geochemical and mineral magnetic characterisation of urban sediment particulates, Manchester, UK

Urban sediments are part of a complex system in which particulates accumulate potentially toxic pollutants, ultimately posing a threat to urban water-bodies and public health. It is therefore important to recognise sources, signatures and pathways of urban particulates. Urban sediment samples were analysed from both inner and outer city road surfaces of Manchester, UK. High metal concentrations, coupled with the largely ferrimagnetic multi-domain (MD) mineral magnetic composition of the particulates, indicate inputs of anthropogenic origin, primarily particulates derived from automobiles, as being the dominant source to the urban sediment system. Iron and Pb concentrations show a clear spatial trend, whereby concentrations are enhanced in the inner city samples. Lead concentrations for inner and outer city samples average 354 and 185 μg g−1, respectively. Iron concentrations for inner and outer city samples average 11302 and 6486 μg g−1, respectively. Sequential extraction analysis shows the metals Mn, Fe, Zn and Pb are largely associated with the reducible fraction, whereas Cu is largely associated with the oxidisable fraction. Zinc is the only metal showing significant association with the exchangeable fraction (up to 33%), suggesting that it may be the most susceptible metal to mobilisation during runoff. Metal fractions identified as showing larger associations with the reducible and oxidisable fractions will continue to be vulnerable to mobilisation as a result of changes in ambient pH and/or redox conditions. Mineral magnetic concentration parameters, such as the saturation isothermal remanent magnetisation (SIRM), together with hysteresis parameters such as the coercive force and interparametric ratios (such as (Bo)cr/Hc and IRM-100mT/SIRM) show that ferrimagnetic MD minerals dominate the Manchester urban sediments. Average results for frequency-dependent susceptibility as a percentage (2%) suggest that the composition of these Manchester sediments is not dominated by soil-derived material, and therefore magnetically coarser, anthropogenically-derived material is dominant. Unlike Pb and Fe concentrations, mineral magnetic characteristics of the Manchester urban sediment samples exhibit no significant spatial variation.

Time-dependent magnetization in fine-particle ferromagnetic systems

Abstract A new theory is presented to describe the time-dependent decay of the isothermal remanent magnetization of a fine-particle ferromagnetic system. Analytical expressions are obtained for a model based on the summation of relaxation times over a very narrow range of the particle size distribution. The model predicts the well established ln t decay of magnetization for small t but a t -1 -dependence for large t . The results are compared with precise experimental data taken on a frozen Co fine-particle magnetic fluid. Excellent agreement between theory and experiment is found over a large measurement time-scale.

Magnetic properties of electrodeposited nanowires

Electrodeposited multilayered nanowires grown within a polycarbonate membrane constitute a new medium in which giant magnetoresistance (GMR) perpendicular to the plane of the multilayers can be measured. These structures can exhibit a perpendicular GMR of at least 22% at ambient temperature. We performed detailed studies both of reversible magnetization and of irreversible remanent magnetization curves for CoNiCu/Cu/CoNiCu multilayered and CoNiCu pulse-deposited nanowire systems with Co:Ni ratios of 6:4 and 7:3 respectively in the range 10 - 290 K, allowing the magnetic phases of these structures to be identified. Shape anisotropy in the pulse-deposited nanowire and inter-layer coupling in the multilayered nanowire are shown to make important contributions to the magnetic properties. Dipolar-like interactions are found to predominate in both nanowire systems. Magnetic force microscope (MFM) images of individual multilayered nanowires exhibit a contrast consistent with there being a soft magnetization parallel to the layers. Switching of the magnetic layers in the multilayered structure into the direction of the MFM tips stray field is observed.

Comparing the resolution of magnetic force microscopes using the CAMST reference samples

A set of reference samples for comparing the results obtained with different magnetic force microscopes (MFM) has been prepared. These samples consist of CoNi/Pt magneto-optic multilayers with di¤erent thicknesses. The magnetic properties of the multilayer are tailored in such a way that a very Þne stripe domain structure occurs in remanence. On top of this intrinsic domain structure, bits were written thermomagnetically using di¤erent laser powers. These samples have been imaged in six di¤erent laboratories employing both home-built and commercial magnetic force microscopes. The resolution obtained with these different microscopes, tips and measurement methods varies between 30 and 100 nm.

Preparation and properties of nickel ferrofluids

Abstract Stable hydrocarbon based nickel ferrofluids have been prepared by ultra-violet decomposition of the tetra-carbonyl or by reduction of nickelocene. Transmission electron diffraction shows the particles to be predominantly fcc nickel with a lattice parameter a = 3.525 A. Electron microscopy revealed median particle diameters below 60 A and relatively narrow size distributions.

High precision torque hysteresis measurements on fine particle systems

The application of an automated torque magnetometer to the precise measurement of torque hysteresis is described and results given for magnetic tapes and colloidal cobalt particles set in epoxy resin.

Determination of the magnetic anisotropy of ferrofluids from torque magnetometry data

Abstract A theoretical analysis is presented which enables the uniaxial magnetic anisotropy constant K 1 of particles in ferrofluids, frozen in an external field, to be obtained from torque magnetometry measurements. The two-fold symmetry of the torque curve, found experimentally, is correctly predicted. An asymptotic solution is found which enables K 1 to be determined without recourse to iterative numerical methods. In this limit, the torque amplitude varies linearly with the inverse of the freezing field for large freezing fields. For all cases, extraction of K 1 requires an accurate knowledge of the particle size distribution parameters.

Torque magnetometry of magnetic fluids

Abstract A sensitive torque magnetometer has been used to measure the anisotropy of Co and Fe 3 O 4 magnetic fluids, which had been previously frozen in fields of up to 0.9 T. The resulting torque curves display a predominant 2 Ψ dependence on angle of rotation and vary systematically with both measuring and freezing fields.

Analysis of radical content on carbon black pigments by electron spin resonance: influence of functionality, thermal treatment and adsorption of acidic and basic probes

The radical content and its change under thermal treatment of different types of carbon black (CBs) are examined using electron spin resonance (ESR) through calibration via stable free radicals. In order to characterise a range of different CBs, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) have been used in an attempt to determine the nature of the functional groups present on their surface. A linear correlation was found between the oxygen content on the CB surface obtained by XPS analysis and the radical content for several grades of pigment. Increasing surface oxygen content resulted in minor changes in radical concentration between the different CB grades analysed. An overall decrease in radical content occurred with exposure of the CBs at higher treatment temperatures accompanied by a recovery between 450 and 750°C. Depending on the CB grade there were also some structural changes, which resulted in an increase in the specific surface area, as well as, a progressively more extensive localisation of the itinerant basal plane electron states associated with chemisorptions of atmospheric oxygen after the thermal treatment. The change of the radical content of the CBs with the thermal treatment was coincident with parallel FMC studies using basic and acidic probes, which revealed changes in acidic and basic character throughout the treatment. These features could be related to changes in surface morphology and chemistry of the modified blacks. Other techniques like thermogravimetric (TGA), BET analyses and Karl Fisher test were also performed in order to determine the differences in specific surface, water and volatile content of the CB pigments and their influence on the radical content. Inter-relationships are shown and discussed.

Preparation and properties of Ni-Fe magnetic fluids

Abstract A heteronuclear organometallic precursor containing Ni and Fe atoms in the ratio 2:1 has been used to prepare hydrocarbon based magnetic fluids. The structure of the resulting magnetic particles has been found by electron diffraction to be that of the ordered Ni3Fe intermetallic. Electron micrographs reveal particle diameters of 7–8 nm. Magnetic analysis indicates diameters of 3–4 nm and anisotropies of ≈105Jm-3.