Perpendicular Magnetization Thin Films And Spin-transfer Torque Effect Simulation Studies

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 Magnetoresistence(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 Spin Transfer torque-induced magnetization dynamics in the spin-valve nanopillars with perpendicular anisotropy:First,we propose a soft/hard in-plane composite free layer design to significantly reduce the critical switching current and switching time;Second,a simple analytical model based on macrospin hypothesis is introduced to interpret the underlying physical mechanism of the reduced critical switching current with the soft nanocore composite free layer;Finally, some RE-TM alloy materials are experimentally tested to be promising candident of the perpendicular magnetic layers.The main contents of this thesis are as follows:In Chapter 1,we briefly review the GMR and STT effect and their application in MRAM.After that,a micromagnetic finite different simulation method used in the thesis is introduced.In Chapter 3,the STT-induced magnetization switching dynamics are investigated in spin-valve nanopillars with perpendicular anisotropy.A promising design with a free layer structure which contains two in-film-plane regions:a main perpendicularly magnetized hard region and a soft nanocore with intrinsic in-plane anisotropy.Micromagnetic simulations show that this design can significantly reduce the critical switching current and improve the switching speed.The magnetization reversal process of the free layer could be divided into two stages:the first stage is for the initial switching from out-of-plane direction to in-plane direction,the second one corresponds to the subsequent switching from the in-plane direction to the final opposite perpendicular direction.The switching is mainly dominated by the competitions among the perpendicular anisotropy field(H_k),demagnetizing field(H_d), and the spin-transfer torque.We find that the first stage is the key for the magnetization switching.The intrinsic in-plane anisotropy of the soft core could not only reduce the effective H_k,but also help draw the moments of the hard region to the in-plane direction through the exchange interaction.Accompanied by the domain nucleation and expansion,a fast switching process with lower switching current could be achieved.In Chapter 4,an analytical model is deduced to describe the current induced magnetization switching in perpendicular spin-valve nanopillar with a soft nanocore. The asymmetric critical switching current,the reduction of critical switching current, and the improvement of switching current symmetry could be well interpreted by combining this theory model and micromagnetic simulations,In Chapter 5,a RE-TM alloys film with perpendicular anisotropy,coupled with a kind of high spin polarization material CoFe,are prepared for the promising candidate of the ferromagnetic layers of the perpendicular MRAM..Finally,the summarization of this thesis and some outlooks of this work are given in Chapter 6.