Theoretical Study Of The Reversal Mechanism Of Exchange-spring And Exchange-bias Systems

Magnetic materials have continuously become more and more important in our lives, and can be found in many common devices such as cars, mobile phones or computers. In this field, exchange-spring and exchange-bias are two interesting phenomena that occur in nanocomposite magnets, with a lot of potential applications. The former appears in hard/soft composite systems, while the latter occurs in antiferromagnetic (AFM)/ferromagnetic (FM) bilayer systems. With a large coercivity Hc provided by the hard phase and a large remanence Mr provided by the soft phase, exchange-spring systems main feature appears to be their high maximum energy product (BH)max.Because of the exchange-coupling occurring between the FM and AFM layers in exchange-bias systems, the hysteresis loop of such a system is shifted from the origin point. This thesis is based on micromagnetics, and investigates the magnetic properties of Nd2Fe14B/Fe65Co35 and Nd2Fe14B/a-Fe exchange-spring systems using OOMMF software and Fortran programming. NiFe/FeMn exchange-bias system is investigated based on a total free energy equation with proper boundary conditions, from which its hysteresis loop as well as other properties can be obtained. The main work is as follows:1. Investigation of the influence of the soft layer thickness on Nd2Fe14B/Fe6sCo35 bilayers, as well as study of the influence of the easy axis angle distribution and interface diffusion on Nd2Fe14B/?-Fe composite systems. We found that the bigger is the soft layer, the higher is the saturation magnetization, and the smaller are the coercivity and remanence. It appears that in exchange-spring systems easy axis angle distribution and interface diffusion phenomena are unavoidable and must be considered when calculating their corresponding maximum energy products.2. Simulation of two models (Mauri's model and HIDW model), based on the principle of energy minimum, permitting the obtaining of NiFe/FeMn hysteresis loops, along with a comparison with experimental data. We also compared the evolution of the energy minimum curve of these two models. In the HIDW model, a domain wall is considered in the ferromagnetic layer. A curve represents the evolution of its thickness according to the external magnetic field intensity.