Spin Transfer Of Moment Modeling The Effects Of Perpendicular Magnetic Anisotropy Devices

Recently, spintronics has been one of hot topics in condensed matter physics due to its wide and promising applications. The Spin Transfer Torque (STT) effect theoretically predicted in 1996 and experimentally demonstrated in 2000 has been considered as another milestone discovery after Giant Magnetoresistance (GMR) effect in the spintronics field. The STT effect provides a convenient and reliable writing scheme of Magnetic Random Access Memory (MRAM), in which the magnetization orientation of a free magnetic layer could be controlled by direct transfer of spin angular momentum from a spin-polarized current. In this thesis, we focus on the study of the magnetization switching dynamics induced by spin transfer torque in magnetic spin valve or tunnel junction based MRAM. Firstly, we study the interactions between the cells of 3 X 3 matrix of MRAM, in which the spin valve with perpendicular anisotropy is used as the memory cell. Secondly, the influences of the perpendicular spin-transfer torque on the magnetization switching of tunnel junctions are investigated. The main contents of this thesis are as follows:In Chapter 1, we briefly review the GMR and STT effect and their applications in MRAM.After that, a micromagnetic finite difference simulation method used in the thesis is introduced in Chapter 2.In Chapter 3,micromagnetic simulation is employed to study the influence of neighboring cells on spin torque switching in a matrix of nanopillars with perpendicular magnetic anisotropy. The neighboring interactions include the stray field and resonant effect between cells. We find that such interaction influence between cells can not be ignored for the high density of MRAM. With the decreased separation, the critical switching current densities can be strongly modified for different matrix configurations as compared to an isolated disk element. Different from the case that the nanopillars have in-plane anisotropy, such variations in nanopillars with perpendicular anisotropy are attributed to the stray fields rather than the resonant effect, although both of them arise from the dipolar interaction between the target and neighboring cells. The Bloch wall mediated switching process is evidenced by the magnetization snapshots, which agrees with the recent experimental results.In Chapter 4, perpendicular spin-transfer torque effect on the magnetization switching was investigated in magnetic tunnel junctions (MTJs). Compared with the in-plane torque, the perpendicular (also called field-like) spin-torque is very small and could be negligible in the metallic structures such as spin-valve or GMR nanopillars. However, the perpendicular spin-transfer torque plays an important role in the magnetization switching of tunnel junctions. We find that effect of perpendicular STT strongly depends on the current direction, showing the larger contributions in the negative current than the positive one. At the given current density, the magnetization switching for the spin-transfer torque with the perpendicular term is faster than that without the perpendicular one. The strength of perpendicular STT increases with the increasing bias voltage. The underlying mechanism can be well explained by nonequilibrium interlayer exchange coupling (IEC) based on majority-and minority-spin bands of the leads.Finally, the summarization of this thesis and some outlooks of this work are given in Chapter 5.