Small Particles System, The Magnetization Reversal And Magnetic Transport

Magnetization reversal of ferromagnetic particles is an important issue in the applications of magnetic data storages, magnetic field sensors and magnetic probes. Techonoledge of manufacturing the magnetic storage and magnetic detector has been well-developed. It makes these devices popular in our daily life. Due to the growth of computing ability and the speed of the central processing unit, the demand of the storage of high access speed is needed. So people are focusing on how to quicken the magnetization reversal and/or decrease the switching field. Furthermore, there is special giant magnetoresistance in the composites of nanosized ferromagnetic particles. The discovery of giant magnetoresistance causes the fabrication of high density memories. As to the composite of nanosized magnetic particles embedded densely in a host, the dipolar interaction between magentic moments will affect the property of spin transport because the orientations of adjacent magentic moments are related to the giant magnetoresistance. It is helpful for understanding the phenomenon of magnetoresistance by studying the property of spin transport in the composite consisting of dense magnetic particles. Meanwhile, our work also gives the theoretical prediction for potential applications. In this thesis, we will study the magnetic property and the spin transport in small particle systems. The details are as follows.We study the magnetization reveral of a nanosized single domain disk by the Object Oriented Micromagnetic Framework (OOMMF) software, which is based on the Landau-Lifshitz- Gilbert (LLG) equation. The single domain particle has a flat shape and its thickness is far less than the length-width sizes. Our previous study showed that the magnetization switching of a circular disk can be quickened in biaxial materials instead of uniaxial materials. However, the crystalline anisotropy depends on the property of material itself. It is difficult to tune the crystalline anisotropy continuously by selecting different materials. Therefore, in this thesis, we suggest to utilize the in-plane shape anisotropy to tune the system's effective anisotropic field so as to reconstruct the potential distribution in space. When the magnetization precesses under the applied field pulse, we find that the critical field that can cause the magnetization reversal and the reversal time are much related to the in-plane shape of the particle. By comparison to the circularly shaped disk, the switching of the properly shaped elliptic disk can be achieved by applying a smaller magnetic field and the magnetization reversal time can also be shortened. By changing the shape of the disk, we can tune the system's effective anisotropic field so as to find the optimal approach to the magnetization switching.We also study the spin transport in the system consisting of magnetic nano-particles. The system that we studied is composed of nanosized magnetic particles regularly embedded in an insulating host. The small particles construct a two-dimensional squared lattice. The mechanism of spin transport in such a system mainly comes from the tunneling magnetoresistance (TMR). Since the dipolar interaction among the particles should be considered, it is difficult to use the LLG equation to describe the instantaneous orientations of the magnetic moments. Here we use the Monte-Carlo method to simulate the orientations of the moments in the lattice. We find that the increase of temperature in a suitable range causes the decrease of TMR. In these composites consisting of dense magnetic particles, the distance between adjacent particles is small and therefore the dipolar interaction plays an important role in the magnetic properties. For a given temperature, the increase of dipolar interaction strength will enhance the TMR and the conductivity of the system will change from a low conductivity to a high conductivity, which is similar to the case of conductivity percolation.