G.N. Coverdale

Time dependence and rate dependence of the coercivity of particulate recording media

The dynamic behaviour of the coercive force of particulate recording media is investigated theoretically. It is shown that as regards the value of Hc the field sweep is equivalent to an effective time determined partially by the particle volume. This is in agreement with an empirical relation arising from recent experimental data.

Modelling of interaction effects in fine particle systems

Abstract A model of magnetisation processes in interacting fine particle systems has been developed. This is based on a Monte Carlo approach and is capable of simulations of temperature and time dependent magnetic properties. A separate model has been developed to predict the microstructure of fine particle systems such as those obtained by the solidification of magnetic fluids. These simulations generate realistic microstructures, which are used as the central cell in the Monte Carlo calculations of magnetic properties. Initial calculations of hysteresis loops and remanence curves have been carried out which demonstrate the importance of the physical microstructure on the interaction problem.

MOLECULAR DYNAMIC MODEL OF THE MAGNETIC PROPERTIES AND MICROSTRUCTURE OF ADVANCED METAL PARTICLE DISPERSIONS

A molecular dynamic model of the properties of a strongly interacting particulate dispersion is described. The model is applied to the study of the properties of a dispersion of advanced metal particles from the intrinsic magnetic behavior through the effects of shear and orientation during the coating process to a final description of the microstructure of the coated medium. The predicted magnetic properties of the dispersion, calculated from the physical properties of the particles and medium, give excellent agreement with experiment. Initial predictions of the effects of shear show the model to be a useful basis for the interpretation of shear magnetometry. Following the “model” coating process the model gives realistic predictions of the microstructure of coated media.

Potential curves and orientational distributions of magnetic moments of chainlike clusters composed of secondary particles

Abstract The present study investigates the potential curves of linear chainlike clusters which are composed of secondary magnetic particles. The orientational distributions of the magnetic moments of the primary particles within the secondary particles are also clarified by means of the usual Monte Carlo method. The results obtained here can be summarized as follows. For the parallel arrangement, in which one cluster lies just beside the other, repulsive forces act between clusters and attractive forces do not arise for any cluster-cluster separation. On the other hand, for the staggered arrangement, in which one cluster is shifted relative to the other in the saturating field direction by the radius of secondary particles, attractive forces do act between clusters at short range. These forces become stronger as the clusters become longer and as the size of secondary particles increases. Although there is a potential barrier for the staggered arrangement, the height of the barrier is almost constant, irrespective of cluster length.

A computer simulation of the microstructure of a particulate dispersion

A computational simulation of a dispersion of iron particles undertaken to study the influence of the magnetostatic interactions on the microstructure of a particle ensemble is reported herein. The simulation considers an equilibrium state derived from an initial random state by the force‐bias Monte Carlo technique. This method favors particle moves in the direction of the magnetostatic forces. A three dimensional ensemble in zero field and a saturating field are studied. An approach which takes into account the magnetostatic interactions between clusters by allowing Monte Carlo moves of whole clusters has been developed. This approach leads to the formation of extended networks consisting of particles in strongly bound clusters which themselves interact and give rise to an extended network. This is similar to the long‐range order observed in practical dispersions. The structure analysis is found to characterize the local order, being especially sensitive to anisotropy in the order produced by an aligning...

Cluster analysis of the microstructure of colloidal dispersions using the maximum entropy technique

Abstract A new method for identifying clusters in magnetic dispersion simulations is presented. The technique is based on maximisation of the entropy of the system. Clusters are generated which are maximally non-committal with respect to missing data resulting in assignments which closely reflect the nature of the association between the objects. By way of illustration, the method is applied to two-dimensional Monte-Carlo simulations of a ferrofluid where the particles are assumed to interact via a point dipole potential. The clusters determined by the algorithm would appear to be perfectly acceptable for this type of interaction. The method is also successfully applied to the more complex case of 3D structures in a strongly interacting particulate dispersion.

A molecular dynamic model of the magnetic properties and microstructure of strongly interacting colloidal dispersions

A molecular dynamic model of the properties of a strongly interacting particulate dispersion is described. The particles concerned are assumed to be acicular and the model is applied to the study of the properties of a dispersion of advanced metal particles for recording media. The model is applied to predictions of the magnetic behaviour of dispersions under shear, providing a basis for the understanding of shear magnetometry. The hydrodynamics of the dispersion are shown to be important as regards the microstructure of the final coated recording medium.

Calculations of the microstructure and magnetic properties of particulate recording media

We report on the hysteresis curves that have been calculated for advanced metal particle media simulated using a multi-stage coating and drying model. The model is a first attempt to simulate a realistic microstructure for particulate recording media from first principles and employs molecular dynamic (MD) and micromagnetic techniques.

Theory of neutron depolarisation in particulate dispersions

Abstract Neutron depolarisation has been simulated to study the microstructure of a particulate dispersion predicted by a Monte Carlo simulation. The zero-field configuration predicts a magnetic correlation length approximately equal to the particle diameter, in agreement with experiment. Moreover, the ND results indicate a random structure of particles.

Computer simulation of the orientation dynamics of a 2D array of fine magnetic particles in an opposed-poles orientation magnet

Abstract The orientation dynamics of a 2D assemblage of interacting acicular particles in an opposed-poles arrangement has been modelled, and is aimed at establishing general principles which will be investigated further by a 3D molecular dynamics model. The equations of motion of the system are represented by a set of three first-order ODEs, where a net torque expression and the infinitely-damped form of the Landau-Lifschitz equation are used to model the particle and moment dynamics, respectively. The resinous vehicle is treated (to a first approximation) as a homogeneous, isotropic Newtonian fluid. This initial study shows the importance of pre-orientation due to mechanical shear during coating.