Theoretical Studies Of The Interfacial Diffusion And Skyrmions In Nanocomposites

Magnetic materials play an important role in the fast development of our modern society, and can be found in many fields, such as aeroplanes, cars or memory devices. Among them, exchange-spring nanocomposites are a new kind of materials which rapidly became popular. It has been widely investigated by scientists and engineers not just because of the combination taking advantage of both the hard and soft phases, but also because of its own good characteristics, such as its excellent stability. Magnetic memory device materials is a significant branch of magnetic materials. Skyrmions, due to their promising potential in this branch of magnetic materials as well as in spintronics, namely, excellent topological stability, very small size and low energy consumption, are expected to be the flagship of a new era of magnetic memory devices. This thesis is based on micromagnetics and used the software OOMMF and the programming language Fortran to investigate the influence of the interfacial diffusion on the magnetic properties of Nd2Fe14B/α-Fe bilayers as well as the behavior of skyrmions in CoPt/Co and CoPt/Co/CoPt. A new formula for the pinning field has been derived. The main work is as follows:1. Investigations of the influence of the interfacial layers on the magnetic properties of Nd2Fe14B/α-Fe systems with in-plane and out-of-plane magnetocrystalline anisotropies, mainly on the hysteresis loops and angular distributions. As the thickness of interfacial layer t1 increases, the coercivity Hc decreases, whereas the remanence and nucleation field barely change. As the applied field reduces, the magnetic moments within the soft phase start to rotate and then drive those of the interfacial layer, meanwhile the magnetic moments within the hard phase keep their coherent state. At the coercivity point, the magnetic moments within the hard phase have a rotation angle of about 30° when t1 is nearly zero, however, the rotation angle is 0° when t1 is larger.2. Investigations of the skyrmions and vortex behaviors in CoPt/Co and CoPt/Co/CoPt without the Dzyaloshinskii-Moriya interaction, as well as study of the influence of material constants on the skyrmion number. Skyrmions are found in both hard and soft phases in bilayer and trilayer systems. As a matter of fact, the skyrmions in the soft phases of bilayer and trilayer systems are 103° and 119° domain walls, respectively. As the anisotropy constant K~h increases, the states of the bilayer and trilayer systems change from the vortex state to the skyrmion state, and then go back to the vortex state.As the anisotropy constant K~s increases, the skyrmion numbers decrease rapidly with the maximum skyrmion number occurrence at K~s=0. The bilayer systems are under skyrmion states when the K~h and K~s are small, and under vortex states when the K~h and K~s are bigger. However, the interfacial exchange-coupling constant Ahs has little effect on the skyrmion number.