Multi - Reflection Spin - Transfer Moment Effect And Its Micro - Magnetism

Spintronics has been one of the hottest topics in condensed matter physics due to its promising potential applications. Spin-transfer torque (STT) effect predicted by Slonczewski and Berger in1996and lately observed in experiment in2000has been considered as another milestone discovery after Giant Magnetoresistance (GMR) effect in the spintronics field. It provides a simple way that the magnetization orientation of magnets could be controlled by a spin-polarized current instead of the magnetic fields. The first product based on the STT effect, namely STT-MRAM, has just come out several months ago. In this thesis, we focus on the studies of the multiple reflection effect in the spin-transfer torque dynamics and the domain wall depinning behavior. We analytically formulate an expression of spin-transfer torque that contains the multiple reflection effect within the interfaces in both the simple "ferromagnet/non-ferromag/ferromagnet" spin valve structure and the complicate multilayers that contains "in-plane and out-of-plane dual spin polarizers". Then the micromagnetic simulations are performed to study the current-driven magnetization switching and magnetization osillcation features. In addition, the mechanism of the domain wall depinning in planar magnetic nanowires with an asymmetric notch has been clarified. The main contents of the thesis are as follows:We first briefly review the GMR and STT effect and their applications in MARM In Chapter1. Then, the micromagnetic simulation method based on the Landau-Lifishitz-Gilbert (LLG) equation used in this thesis is introduced in Chapter2. In Chapter3, by using a scattering matrix method, an analytic spin-transfer torque model that contains the multiple reflection effect has been addressed. In comparision with the common Slonczewski STT model, micromagnetic simulations based on the new model show that the multiple reflections could reduce the critical switching current. In addition, we found that the spin polarization of the two ferromagnetic layers could influence the critical current. For the magnetization from parallel to antiparallel switching, the critical current could increase (decrease) with the increase of the polarization of the free layer (pinned layer). However, for the antiparallel to parallel switching, the critical current decreases for increasing the spin polarization of either the free layer or the fixed layer.In Chapter4, we further expand the STT formula with multiple reflection effect to the spin valves with the in-plane (IP) and out-of-plane (OP) dual polarizers. It is found that the multiple reflections could induce asymmetric dependence of the oscillation frequency versus current even if the three ferromagnetic layers have the same parameters, showing the frequency in the negative current sense varies much faster than that in the positive current case. This asymmetry is attributed to the angular dependence of the spin torque factor. Besides, not only the polarization of OP layer but also the polarization of free layer and IP layer make contribute to frequency of oscillation.In Chapter5, we investigated depinning effect of domain wall (DW) in NiFe planar magnetic nanowires with an asymmetric notch. It is found that the depinning fields strongly depends on the charility of the vortex DW.