Spin Polarized Transport In Ferromagnetic / Metal Tunnel Junctions

Recently, with the application and development of the nano-technology, spintronics has become a fast developing area. It is a new cross-field based on the study of mesoscopic and micro-scale range of spin-polarized electron transport properties (including spin polarization, spin scattering and spin relaxation, etc). The spin-polarized transport of electrons has become one of the important aspect of condensed matter physics. In this dissertation, we study the spin-polarized transportation and the TMR effect in the ferromagnet/ half-metal tunnel junctions.We first introduce the background of the tunneling magnetoresistance effect; then two familiar theoretical models and their calculated methods are presented, such as Julliere model and the tunneling Hamiltonian method, Slonczewski model and the quantum-mechanical tunneling method.Following, we study the spin-polarized transportation in the ferromagnet/ half-metal tunnel junction, here adding the half-metal layer is to enhance the polarization of electrons. It is found that the transmission coefficients of spin-up and spin-down electrons show typical resonant transmission properties. Moreover, when the strength of potential barrier, thickness and the split energy between spin-up and spin-down subband of half-metal layer increase, the TP↓↓and TAP oscillate more rapidly while the TP↑↑is nearly invariability, and the TMR effect of the junction also improved. So the half-metal layer applied is very useful to enhance the TMR effect. At last, a double tunnel junction built by the ferromagnet/half-metal, where high degrees of spin-polarized electronics are injected into the basic junctions by inserting a half-metal layer in the FM/I/I/FM junction, have been investigated. We discuss the resonant tunneling and the TMR effect in the junction by a scatter approach called quantum-mechanical approach. The results show that the transmission coefficients in the ferromagnetic half-metal double tunnel junction has similar characteristics as that in simple ferromagnetic half-metal tunnel junction, such as resonant transmission properties, etc. But the resonant peaks become sharper when the insulator layer is added. Moreover, when the strength of potential barrier in insulator increases, the maximum value of TMR becomes larger, then the spin-filtering effect become more obvious. So we can get the needed TMR by selecting suitable magnetic parameter for ferromagnet/half-metal tunnel junctions. Our above work is expected to be helpful for the developing of new magneto-resistance devices and the research about the tunneling effect in quantum mechanics.