A. Bradbury

Agglomerate formation in a magnetic fluid

Monte Carlo techniques have been used to investigate the effects of magnetostatic and repulsive particle interactions on the formation of agglomerates in a magnetic fluid. The dependence on particle size and applied field of the form of the agglomerates was studied using a spatial distribution function which allows a quantitative distinction to be made between clusters and anisotropic chain structures. Magnetization curves have been calculated for magnetic particle sizes varying from 5 to 15 nm with and without magnetostatic interactions. For the larger particle sizes, it was found that the initial susceptibility is reduced in the presence of interactions. This is associated with the presence of pronounced agglomeration in zero field, where open clusters are formed. As the applied field is increased the clusters break up to form long chains aligned in the field direction. At intermediate particle sizes, there is evidence of magnetic field induced agglomeration leading to the formation of dimers and trimers preferentially aligned in the field direction. The smallest particle size showed little evidence of ordering even in strong applied fields, since thermal disordering dominates the situation.

Particle size analysis in ferrofluids

Abstract In this paper we examine the applicability of the Gaussian and lognormal probability functions to describe the distribution of particle sizes found in ferrofluids. Measurements have been made of the particle size distributions contained in a large number of ferrofluids prepared by different techniques. From these measurements we conclude that the form of the distribution may be associated with the technique of particle preparation.

Particle cluster configuration in magnetic fluids

A model based on Monte Carlo methods has been used to investigate the effects of magnetostatic and repulsive interparticle interactions on the properties of a magnetic fluid. The model predicts the formation of open loop structures in the absence of a magnetic field, and long chains in the presence of large magnetic fields. The model also predicts that the initial susceptibility is reduced in the presence of interactions. These predictions are in agreement with experimental observations.

Interaction effects in longitudinally oriented and non-oriented barium hexaferrite tapes

The observed magnetic interactions in barium hexaferrite tapes as a function of dispersion milling time and orientation are reported. Measurement of the interaction state is obtained by comparing the magnetizing and demagnetizing remanence curves. Variation in the interactions as a function of dispersion milling time in longitudinally oriented and nonoriented tapes indicates substantial changes in the interaction state of the constituent magnetic particles. In particular, the oriented tape samples exhibit an increasing net magnetizing interaction effect, whereas the nonoriented samples show a decreasing net magnetizing effect. These two markedly different effects are explained in terms of the improved dispersion and separation of barium hexaferrite platelets, which reduces the interaction parameter in the case of nonoriented tapes. In the longitudinally oriented form, however, the increased freedom of movement (associated with improved dispersion) results in the enhanced formation of stacking and particle columns. >

Curie-Weiss behavior in ferrofluids

Abstract Curie-Weiss behavior has been shown for magnetic liquids containing cobalt particles diameter D vm = 27 A (sample A) and D vm = 96 A (sample B). Values for an internal field of 650 Oe and 100 Oe have been calculated for samples A and B respectively which is interpreted in terms of interparticle interactions. Monte Carlo calculations give some justification for this approach.

Interaction effects in the remanence curves of CoTi-doped BaFe systems

Measurements are reported on doped barium ferrite particles produced through chemical coprecipitation. The data are analyzed using a parameter Delta M(H). The behavior of Delta M(H) is reported to be strongly dependent on the doping and consequently the coercivity of the media. High-coercive-force media exhibit a net magnetizing effect in Delta M(H), which changes to a net demagnetizing effect as the coercivity decreases. This is interpreted in terms of changes in the particle interactions introduced through doping. >

A model of the properties of colloidal dispersions of weakly interacting fine ferromagnetic particles

Abstract A Monte Carlo technique has been used to simulate the magnetic properties of a colloidal dispersion of weakly interacting fine ferromagnetic particles. The initial susceptibility is shown to obey a Curie-Weiss like law in its variation with temperature. The ordering temperature in the Curie-Weiss law is found to increase with the diameter of the particles, the increase being associated with an increase in the local order in the system. Data from the Monte Carlo simulation is also used to assess the effects of interactions on the determination of particle size parameters from magnetic measurements. Investigation of the spatial correlation within the system reveals evidence of field induced particle agglomeration.

Magnetic size determination for interacting fine particle systems

Interparticle interactions play an important role in the magnetic response of a magnetic fluid. Using the Monte Carlo technique, the effects of interactions on the determination of magnetic particle size distribution parameters are investigated. It is shown that these interactions introduce a significant error in the value obtained for the standard deviation, whilst the effect on the median diameter is less pronounced. Using an approach based on the representation of interactions by means of a thermodynamic ordering temperature, expressions are derived which enable the distribution parameters of a weakly interacting system of particles to be determined. These equations are applied to experimental data for two different types of colloidal dispersion, in each case giving improved results.

Monte Carlo simulations of the structure of magnetic fluid composites

Monte Carlo simulations have been used to determine the structure of thin film composites containing polystyrene spheres 1 mu m in diameter dispersed in a 50 kA m-1 magnetic fluid. The aggregate structures observed in a magnetic field of 2 mT correspond closely to those which are observed experimentally for both high (16%) and low (6%) concentrations of spheres. The analysis predicts a variation in structure with magnetic field orientation, giving a change from a regular hexagonal lattice structure when the field is perpendicular to the plane of the film to a system of chain aggregates when the field is at an angle of approximately=60 degrees to the plane of the film.

A Monte Carlo calculation of the magnetic properties of a ferrofluid containing interacting polydispersed particles

Abstract Monte Carlo simulations of polydispersed ferrofluids have been made. For small median particle diameters the magnetisation is enhanced due to short range order involving large particles only. In the more strongly interacting systems all the particles are ordered with more densely packed aggregates than was found previously in monodispersed systems.