Models of microstructure and magnetic properties for magnetic recording media
Three computational models have been developed to simulate magnetic properties of granular media, particulate media microstructures and self-assembled systems. The granular media model uses an energy minimisation approach to describe the magnetic properties of a system of randomly oriented single-domain particles taking into account dipolar and exchange interactions as well as thermal effects. At low temperature dipolar interactions produce flux closure vortex structures leading to a decrease of both remanence and coercivity. When thermal effects become important, dipolar interactions lead to an increase of the local energy barriers increasing both remanence and coercivity as compared to the superparamagnetic case. Exchange coupling tends to align the magnetic moments producing an increase in the remanence of such systems while cooperative reversals decrease their coercivity. The particulate media model uses a spherocylindrical approximation for the elongated magnetic particles that are used in tapes. The particles are allowed to move in a viscous solvent under the action of steric and magnetic interactions and of the orienting field. A percentage of the particles are grouped in clusters that behave as rigid bodies during the simulation. The results obtained suggest that the presence of the clusters leads to a disruption in the alignment of the free particles regardless of the cluster size. A third model uses a Monte-Carlo approach to describe the self-assembly process that occurs in surfactant coated magnetic particles. As the solvent dries the particles form assemblies to minimize the interaction energy. In order to obtain long-range self-assembled systems the particle areal density must be in a narrow range and the particle size distribution must have a standard deviation below 5%. The occurrence of local self-assembly is due to the presence of an attraction term in the interparticle interaction potential. The conditions under which square vs. hexagonal lattice can be obtained are discussed.