The Micromagnetic Investigation Of Exchange-spring And Exchange-bias Magnetic Multilayers

Exchange-spring multilayers, with their high coercivity and high remanence provided by hard and soft phases, respectively, are excellent candidates for permanent magnets and storage materials. The reversal process and coercivity mechanism of such magnets have attracted much attention recently. In this thesis, the nucleation field and magnetization angular distribution of exchange coupled multilayers are calculated analytically within a micromagnetic method. It is found that the nucleation field decreases smoothly from the anisotropy field of the hard phase to that of the soft phase as the soft-layer thickness increases. The platform or peak observed in the curve of nucleation field by other groups doesn't occur in our results. Detailed analysis shows that our calculation is more reliable and agrees better with available experimental data. The nucleation field increases as the hard-layer thickness increases, however, this effect is obvious only when the thickness of the hard phase is very small. At the nucleation state, only the relative angular distribution could be determined. After the nucleation, however, the angular distribution could be determined completely, which has been calculated and given as a function of applied field for perpendicularly oriented Nd2Fe14B /α-Fe trilayers. The hysteresis loops of such trilayers have also been calculated with demagnetization energy taken into account. For small Ls, the theoretical coercivity equals to the nucleation field (it also equals to the pinning filed). For large Ls, however, the coercivity is slightly smaller than the pinning field. As the thickness of the soft phase increases, the remanence has a peak and the coercivity mechanism changes from nucleation to pinning. The methods and conclusions adopted here are also valid for other exchange-spring multilayers.Exchange-bias(EB) of ferromagnet / antiferromagnet systems is another hot topic in magnetism. Lots of models have been proposed to explain the mechanism of EB, but all of them have some defects. In this thesis, the mechanism of EB has been explored based on a competition between rotational and pinned spins in the antiferromagnet. It is found that available models of EB could be regarded as some sort of nucleation mode. The rotation-pinning interface is determined by the minimization of the nucleation field. A model with exponentially decaying magnetic parameter in the interface has been proposed. The nucleation fields have been derived and compared for complete and partial coherent-rotation models. The formulas for exchange-bias and coercivity have been obtained which agree well with experiment. The present approach offers a new insight into the mechanism of exchange bias.