D.A. Parker

Comparison of the effects of regular and irregular physical structure on the magnetic microstructure of longitudinal thin films

Abstract Interaction effects in longitudinal thin films are investigated by computing δI curves formed by comparison of the remanence curves using the Wohlfarth relation. Historically, throughout the literature, interpretation of magnetic microstructures has relied upon visual inspection of vector maps representing such configurations. However, in this paper, theoretical techniques are used to analyse the magnetic microstructures of simulated AC-erased and DC-demagnetised states; these take the form of a correlation function and Fourier analysis. The role of physical structure on such interactions is investigated by comparing the simulated curves generated from a system of grains lying on a radially isotropic structure with those of a system whose physical structure forms an hexagonal closed-packed (HCP) lattice. Different degrees of exchange coupling are considered. Disorder in the microstructure is shown to be responsible for a reduction in the magnitude of positive δI values. It will be shown how the degree of cooperative switching during the magnetisation process is dependent on the underlying physical structure of the systems.

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 model of an isolated column of metal evaporated tape

Metal Evaporated (ME) tape comprises bundles of inclined columns each of which consists of small crystalline gains. To simulate the magnetic behaviour of ME tape we have developed a model, using a Monte Carlo algorithm to represent a single column of spherical grains whose radii are generated from a log-normal distribution. The dynamic process of the magnetic moments is explored by utilizing the Landau-Lifschitz equations. We demonstrate that the microstructure of the column and direction of the easy axes influence the magnetic reversal process. By varying the strength of the exchange coupling parameter its effect on the amount of co-operative switching is reported. The results suggest that high exchange coupling acts to keep moments aligned, resulting in higher values of coercivity. The reversal mechanism is initiated at the ends of the column and then propagates through the column.

A simulation of the role of physical structure on feature sizes in exchange coupled longitudinal thin film

Abstract A model has been developed which simulates magnetic reversals of grains arranged on an irregular physical structure. A correlation function and a spectral analysis technique are used to compare the effects of physical structure on magnetic microstructure.

Interaction effects from reversal studies of single particles

Abstract Interaction effects within particulate recording tapes have been investigated by comparing single particle behaviour with measured tape bulk properties. The particle systems were audio γ-Fe2O3 and 8 mm MP video tapes plus CrO2 hand spreads. Interactions in the γ-Fe2O3 and CrO2 systems were negative and consistent with mean field behaviour. However, the MP particle interactions were in the opposite sense, in contradiction to the Henkel plots. A model for this behaviour involving the creation of chains is suggested.

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.

A micromagnetic study of the reversal processes in metal-evaporated tape

The morphology of ME tape undoubtedly affects its magnetisation reversal processes. It is therefore essential that a realistic model is used when simulating the hysteresis of the film. A detailed 3D model representing the columnar structure of ME tape is developed here and the reversal processes, in an assembly of columns, are investigated numerically. The results demonstrate that reversal is affected by the strength of the exchange coupling parameter. Incoherent reversal occurs for weak coupling whereas increasing the strength brings about a high degree of co-operative reversal within columns. Strong exchange coupling and column demagnetising effects cause the moments to initially relax towards the long axes of columns. Then as the applied field is reduced the moments reverse at the column ends to form closure domains. These results indicate a subtle relationship between magnetostatic and exchange interactions occurs during reversal.

A micromagnetic model of metal-evaporated tape

A 3D micromagnetic model of metal-evaporated tape has been developed to represent the granular nature of the film. The model simulates the columnar structure of ME tape by fitting together columns of spherical grains whose radii have been generated from a log-normal distribution. It is demonstrated that for high exchange coupling there is a high degree of co-operative reversal in the columns; whereas for little or no exchange coupling reversal is by incoherent rotation. For high exchange coupling co-operative reversal causes regions of flux closure due to strong demagnetising fields.

Angular Dependence Of Magnetisation Reversla in /spl gamma/Fe/sub 2/O/sub 3/ Single Particles - An Experimental And Modelling Study

The switching field for isolated /spl gamma/-Fe/sub 2/O/sub 3/ magnetic particles has been investigated as a function of the applied field angle both experimentally and numerically. Micromagnetic simulations for a typical measured particle with cubic crystalline directions of and with respect to its long axis were compared directly with experimental results. At large applied field angles, the agreement between experimental results and numerical simulations was poor. However, at the smaller applied field angles the simulations gave reasonable agreement with the experimental results.