The Theoretical Investigation Of Exchange-coupled Two-phased Nanocomposite Permanent Magneticfilms

Exchange-coupled two-phased nanocomposite magnets, with a hard phase to provide high coercivity and a soft phase to provide high saturation, are widely regarded as the excellent candidates for permanent magnetic materials. In this thesis, the nucleation field, the pinning field, the coercivity mechanism and the hysteresis loops of the exchange-coupled two-phased nanocomposites have been investigated within a micromagnetic method.1.The demagnetization process of a hard/soft/hard sandwich has been investigated systematically, with particular attention on the cases with small hard layer thickness. The hysteresis loops, as well as the angular distributions of the magnetization between nucleation and pinning have been obtained numerically, with the formula for the nucleation field derived. It is found that both nucleation and pinning fields, as well as the gap in between decrease as the hard layer thickness reduces. The hard layer thickness has great effect on the hysteresis loops only when the thickness is very small, where the hysteresis loop is nearly square and the dominant coercivity mechanism is the nucleation. The thickness regions at which the theoretical and practical giant energy products can be achieved have been discussed. In most cases of experiments, the hard layer can be taken as sufficiently thick so that the magnetization at its surface obeys a simple coherent rotation model. In these cases, the calculation can be simplified significantly, with only the influence of the soft layer thickness accounted.2.Using the interface coupling constant and the soft layer thickness as the main variables, the changes of the magnetic moments with the applied field and the hysteresis loops of Nd2Fe14B/α-Fe trilayers, have been investigated. When the soft layer thickness is smaller, the pinning field equals to the nucleation field, where the coercivity mechanism transforms from nucleation to pinning as the interface coupling constant Ji decreases, whereas for large soft layer thickness this trend is reversed. The critical thickness, at which the nucleation field and pinning field detaches, decreases as Ji decreases. When the reduced exchange coupling is considered, the"Rigid Composite magnet"appears only when the soft layer thickness is very small. The reduced exchange coupling leads to a gap of angle of magnetization at the interface, which results in the changes of the behavior of the trilayers from the single-phase one to the two-phase one and that the nucleation field decreases while the nucleation field increases when the soft layer thickness is larger.3.Taking the Nd2Fe14B/α-Fe, FePt/α-Fe and SmCo5/Co magnetic trilayers for example, the influence of the strength of interface exchange coupling as well as the soft and hard layer thickness on nucleation field has been studied systematically. Analyses show that the interface coupling constant Ji has significant effect on the nucleation field only when the soft layer thickness is about 10% of the domain wall width of the soft phase while the thickness of the hard layer is larger than its domain wall width. The hysteresis loops of the Nd₂Fe₁₄B/α-Fe/ Nd₂Fe₁₄B trilayers for various Ji have been calculated. It is found that, as the soft layer thickness increases, the coercivity decreases and the coercivity mechanism changes from pinning to nucleation. The theoretical and experimental loops of Sm40Fe60/Ni₈₀Fe₂₀ bilayer agree quite well when the value of Ji is taken as10% of Jibulk(the intermediate value between the two interface coupling constants corresponding to the soft and hard layer respectively), indicating that the interface coupling in the experiment is poor.