单畴磁颗粒的磁矩动力学研究

【作者】 王志远；

【导师】 孙周洲；

【作者基本信息】 苏州大学 ， 光学， 2014， 硕士

【摘要】 研究磁性材料的磁矩翻转问题是一个非常重要的课题，因为它被广泛的应用在了磁性存储器件中，与人们的日常生活密切相关。目前随着信息时代的飞速发展，磁性存储器件的存储密度、存储速度越来越难满足人们的需要，因此对高密度磁性存储技术以及快速信息存储与读取技术需要不断的发展。任意材料的磁矩翻转都是一个极其复杂的过程，使用简化的模型来研究磁矩翻转不但可以大幅度节省时间，同时给实际的应用带来很好的指导作用。本论文主要对斯托纳粒子的磁矩翻转动力学进行研究，从两个方面探讨了单轴模型下单畴粒子的磁矩翻转。文章的架构如下：第一章为引言，是通过对已有的重要文献的解读对本领域的研究背景和发展近况简单地做一个介绍。第二章为理论基础，主要介绍了基本方程，即朗道-利夫希茨-吉尔伯特（LLG）方程以及其球坐标形式。并且介绍了含有自旋转移矩项的Landau-Lifshitz-Gilbert-Slonczewski（LLGS）方程。第三章讨论的是含时磁场作用下的磁矩翻转，磁矩动力学过程满足朗道-利夫希茨-吉尔伯特（LLG）方程。从这个方程出发，用理论解析、数值求解的方法对斯托纳粒子的磁矩翻转进行了分析。当所加外场为随时间正弦变化的磁场时（例如电磁波或激光），若保持磁矩翻转的时间等于外场变化的半周期时间，则所加外场的振幅与频率必须维持一个恒定的比率。然后我们采用一种整流型脉冲磁场作为驱动场，我们就能够方便地通过调节脉冲数来降低所需振幅或者缩短磁矩翻转时间。论文第四章对自旋转移矩（STT）驱动的磁矩翻转进行了研究，以Landau-Lifshitz-Gilbert-Slonczewski（LLGS）方程为出发点，主要对任意角度的极化电流驱动的单磁粒子的磁矩翻转进行稳定性分析。只考虑单轴各向异性，磁化矢量的平衡态解可以由一个一元三次方程解析求出。我们发现，对应于不同的电流极化方向与电流强度，可能存在一至三对平衡态，通过数值求解平衡态的稳定性，可以得出任意极化角度下磁矩翻转所需要的阈值电流，这对今后STT驱动的磁性器件的设计有着重要的指导作用。第五章是对整个文章的总结，并且提出一些遗留问题以及展望。更多还原

【Abstract】 Studying the magnetization reversal of the magnetic materials is a very importantsubject because of its widely use in the magnetic storage devices, and it is closely relatedto our daily lives. Now with the rapid development of information age, the storage densityand speed of the storage is more and more difficult to meet the needs of people. So thestorage technology of high density and speed need the continuous development. The realmagnetic moment reversal is a very complicated process, thus we should use the simplifiedmodel to study it to save time and also bring good guidance to the practical application. Inthis thesis, the magnetization reversal dynamics was studied. The magnetic momentreversal of the single-domain particle in the uniaxial model was discussed from twoaspects. The structure of this article is as follows:The first chapter is the introduction of the background and development of this fieldby interpreting existing references.The2ndchapter is the theory of our work. In this part we have a introduction of theLandau-Lifshitz-Gilbert equation and the spherical coordinates form of it. Also the LLGequation with the spin transfer torque item was introduced.In chapter3, we have discussed the magnetization reversal under the time dependentmagnetic field. The dynamics can be described by the Landau-Lifshitz-Gilbert equation.From this equation, both the analysis and numerical methods were used for studying themagnetization reversal of the Stoner particle. When use a sinusoidal variational magneticfield as the external field (like electromagnetic wave or laser), there existed a fixed ratiobetween the amplitude and frequency of the external field to maintain the reversal timeequal to one half period of the applied field. Then we used the continuous pulsed rectifiedmagnetic field pulse as the external field. Thus one can conveniently decrease theamplitude or increase the reversal speed by adjusting the number of the pulses.STT-induced magnetization reversal was discussed in chapter4. The stationary-statesolutions of magnetization dynamics under a spin-polarized current that was polarized inan arbitrary direction were investigated by solving the Landau-Lifshitz-Gilbert-Slonczewski equation for a single-domain magnet. Taking into consideration the uniaxialmagnetic anisotropy, the equilibrium directions of the magnetization vectors were analytically obtained by solving an algebraic cubic equation. It was found that one to threepairs of magnetization equilibrium states existed, depending on the current intensity andthe direction of the spin polarization. By numerically analyzing the stabilities of theseequilibrium states, the threshold switching current for the reversing the magnetic vectorwas obtained under different current polarization configurations, which may be useful foruse in future spintronics devices.The last chapter is the conclusion of this article. And some leftover problems were putforward. We also give some outlooks at last.更多还原