The Micromagnetics Investigation Of Two-phased Nanocomposite Permanent Materials

The composite magnetic materials, which comprise a hard phase to provide high coercivity and a soft phase to provide high saturation and remanence, have been a hot topic in both theoritical and experimental study. The increasing of magnetic density for material becomes a kind of demand with the development of the information society, and the vertical magnetic recording technology is a good method to improve the magnetic density. In this paper, the coercivity mechanism and the magnetic reversal process have been studied for the perpendicularly oriented composite magnetic system within a micromagnetic method with high density longitudinal magnetic recording media. The main research is as follows:1. Nucleation, coercivity mechanism and the magnetic reversal process in perpendicularly orientated trilayers have been investigated using Nd₂Fe₁₄B/α-Fe as an example, concentrated on the situation with very thin hard layer. The formula for the nucleation field has been derived. When the hard layer is smaller than its Block wall, the influence of the hard layer on nucleation field is much obvious. Negative nucleation field occurs at thick soft layer when the shape anisotropy surpasses the crystalline one. The effect of the hard layer on nucleation field is similar when the hard layer is larger or smaller. The coercivity mechanism changes from nucleation to pinning as the soft layer decreases. However, for larger soft layer and smaller hard layer, the magnetic moment in the soft layer obeys to the coherent rotation mode whilst the hysteresis loop approaches linear.2. The interface exchange coupling on the magnetization reversal process for perpendicularly orientated tri-layer structure has been studied using FePt/α-Fe of the high coercivity and magnetic density material as an example. The nucleation field increases with interlayer exchange constant. Especially when the soft layer is small, the effect of the interface exchange coupling on nucleation is apparent, however, the influence could be neglect for larger the soft layer. The pinning field decreases as the interlayer exchange constant rises. The magnetic moment angle has a gap when interface exchange coupling becomes weak. As the soft layer increases, the coercivity mechanism transforms from nucleation to pinning. But when the strength of interface exchange coupling is weak, the center of domain wall pins the hard/soft interface rather than in the hard layer. In the meanwhile, comparison of the critical thickness corresponding to coercivity, the perpendicularly orientated case is much smaller than the parallel orientated case, coinciding with the result of the numerically calculation.