Micromagnetic Simulation For Low Dimension Nanoscale Magnetic Materials

In this thesis, macroscopic hysteresis loops, microscopic magnetic moment distributions, nucleation, coercivity, and energy product have been determined by three-dimentional(3D) micromagnetic models for exchange-coupled Nd2Fe14B/Fe65Co35 bilayers with a deviation of easy axis β taken into account, and carefully compared with experiental results. We find that nucleation only occurs at β =0o. The nucleation and coercive fields decrease monotonically as the soft layer thickness Ls increases whilst the largest maximum energy product(586.4 k J/m3) occur at Ls = 3nm,β = 0o. Compared with the experienmental results, one can find that the remenance and maximum energy products are larger than those in experiment. For Ahs = 1 × 10-11J/m, the coercive field is 1.35 T, which is smaller than 1.77 T in experiment. For Ahs =0.3 × 10-11J/m, it is 1.82 T larger than that in experiment.Besides, the magnetization reversal in of cobalt nanowires has been studied. It is found that the geometrical size has a significant effect on the coercivity of the nanowires. With increasing aspect ratio p, the coercivity of cobalt nanowires increases gradually due to the shape anisotropy. Also our calculations have demonstrated that the coercivity can be largely reduced by the non-coherent orientation of the easy axis with the direction of the cobalt nanowires. In addition, four types of reversal modes including coherent quasi-coherent rotation, nucleation, anti-curling, and curling have been determined by carefully analysing the energy terms describing the system.dynamic magnetic susceptibilities of the vortex state in permalloy nanodot arrays have been investigated using a three-dimensional Object Oriented Micromagnetic Framework(OOMMF) code with a two-dimensional periodic boundary condition(2D-PBC) extension and compared with those of a single dot carefully. The influence of the parameter variation on the resonance frequency has been studied systemically,including the dot radius, the number of repeating elements, and the dot distance. One can see that the resonance of dot array is higher than that of a single dot because of the induced stronger magnetostatic coupling. A critical dot distance exists at which the dot array may be treated as a single dot. Also a peak resonance value occurs with the radius increasing for both dot array and single dot.