J. Popplewell

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 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.

Time dependent magnetization of a system of fine cobalt particles

Measurements are presented which show that the magnetisation of a system of fine cobalt particles at 77°K is time dependent. The system examined in this work is of fine cobalt particles with median diameter D v = 70 A dispersed in toluene. The particles have been found to be in the fcc phase. The time decay of magnetisation has been found to be logarithmic. The time dependence coefficient A = -d\bar{I}/dLnt varies with the applied field H and is a maximum for H = H c the coercive force. These observations are in good agreement with the theory presented in this paper.

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.

The low temperature magnetisation of a system of fine cobalt particles

The low temperature magnetisation of a system of fine cobalt particles dispersed in toluene has been measured. The remanence decays logarithmically with temperature. From these results a value of the effective anisotropy constant K = (1.9±0.5) × 106ergs/cc (at 40K) is obtained. Theory is presented to show that the magnetisation on approach to remanence is linear with H. From the gradient a second estimate of the anisotropy constant can be obtained at 77K having a value of K = (1.6±0.5) × 106erg/cc. These values which are much larger than would be expected for f.c.c, cobalt are consistent with the presence of dominant uniaxial shape anisotropy. This conclusion is supported by the observed remanence to saturation ratio of 0.5 at 0 K.

Initial susceptibility of ferrofluids

Abstract From measurements of the variation of the low field susceptibility with temperature, it has been found possible to determine the dominant mode of magnetisation in ferrofluids.

Effects of anisotropy field distributions on magnetic viscosity in particulate recording media

Theoretical predictions are made of the time-dependent behavior and the irreversible susceptibility of a system of uniaxial, single-domain particles. These results are used to predict the fluctuation field H/sub f/. These predictions are consistent with experimental data on particulate recording media. >

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.

Determination of easy axis distribution in recording media

A method is reported for measuring the effective easy axis distribution of a two-dimensional system, which is more applicable to recording media. The method is applied to cobalt-phosphorus thin films and to a conventional particulate tape. The method provides a useful quantitative analysis of magnetic texture. The measurement displays clearly the effects of interactions in the thin films. These interactions are not so great in the case of particulate media. >

Components of magnetisation of a fine particle sytstem

A detailed study of the different magnetization components in a fine particle system is presented. From measurements of these components, it is shown that the various components are related. The effects of particle interactions on the magnetic behavior of the system are also discussed. This behavior is shown to be analogous to that of spin-glass alloys. Low-field measurement are greatly affected by particle interactions, whereas interactions have little effect on saturation measurements. >