S.W. Charles

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.

The isothermal remanent magnetisation of fine magnetic particles

The effect of a finite maximum applied field on the remanence of a fine-particle system with uniaxial anisotropy and easy axes aligned parallel to the applied field is investigated. Theoretical calculations are presented for iron, cobalt and Fe3O4 particles, and it is shown that large values of remanence are obtained for large values of the product KV (K is the anisotropy constant and V the particle volume). For practical purposes a technique is outlined whereby optimum values of K and V may be chosen in order to maximise the remanence of the system. Measurements are presented of the remanence of a system of fine cobalt particles as a function of the maximum applied field and a comparison made with the theoretical predictions.

The effect of field induced texture on the properties of a fine particle system

Abstract We have measured the magnetic properties of textured fine particle systems consisting of cobalt particles in a solid environment. The texture of a system refers to the alignment of anisotropy easy axes within the composite which is brought about by solidifying the sample in a large applied field. It is found that the texture of the irreversible particles is reflected in the remanence of the system while that of the reversible particles is evident in the slope of the magnetisation curve in small fields. Theoretical calculations give good agreement with the experimental data.

The coercivity of a system of single domain particles with randomly oriented easy axes and a distribution of particle size

Abstract The magnetization curve at 77 K for uniaxial cobalt particles with easy axes randomly oriented is shown to be in agreement with a theoretical curve calculated for a particle size distribution with a median diameter D v = 63 A, σ = 0.3 and anisotropy constant K = 2.8 × 10 6 erg cm -3 . The values are consistent with magnetic measurements at 300 K and microscopy data.

The isothermal remanent magnetisation of particulate media

A model is presented of the remanent magnetisation of a system of non-interacting Stoner-Wohlfarth particles with differing degrees of orientational texture. IRM curves are calculated which are used ho show the effect of a finite magnetising field on the determination of optimum values of diameter and anisotropy constant. The shape of the SFD is also found to depend strongly on the degree of orientational texture. The width of the SFD is increased or decreased relative to the aligned case depending on whether the intrinsic SFD for the aligned system is small or large respectively.