【作者】 刘斌；

【导师】 刘喆颉；

【作者基本信息】 太原理工大学 ， 电子科学与技术， 2015， 硕士

【摘要】 随着科技的发展和大数据时代的到来，信息已经成为人们正常生活中不可分割的一部分。大数据的记录使得小器件、大容量的存储器件成为大众所追求的目标。因此，改善和提高存储器件的密度具有十分重要的意义。由于垂直结构的自旋转移矩磁阻式随机存储器（STT-MRAM）具有非易失性和高速无限写入循环等优势，而被研究人员认为将会成为主流的通用存储器。在磁性纳米结构内，磁矩翻转动力学特性是反映存储器件性能的直接指标，该特性的研究对磁存储以及自旋电子学都具有十分重要的意义。然而，由于以往使用面内磁结构，为了达到降低电流密度的目的，所制作的磁存储器件截面积较大，因此通常在使用宏自旋模型对STT-MRAM的分析中采用简化的退磁因子，即只考虑垂直方向退磁因子影响（或是假定截面积无穷大）。但是在垂直磁结构中，翻转电流密度较小，在不失热稳定的前提下可以进一步减小其截面积，这种情况下如不考虑横截面积的影响就可能带来较大误差。同时，之前的研究表明，通过改变自由层或极化层的厚度，如改变CoFeB薄膜或者Co/Pd多层膜的厚度，可以使磁矩产生一个小的倾角，但在尺寸有限的情况下该倾角的大小对磁矩翻转的影响尚不明确。为此，本文围绕磁矩翻转的动态特性作了如下工作：1.介绍了磁存储的意义、发展历程、当前现状和未来发展方向。2.介绍了退磁场及退磁能，并采用磁偶极子相互作用的观点对退磁场进行了详细的推导计算，同时对椭球体与矩形结构中各分量退磁因子作了归纳总结。3.深刻阐述了宏自旋模型的处理方法，并列出了其优缺点。尤其通过数值模拟的方法，着重对自旋转移矩（STT）效应的工作机理及作用效果进行了说明。4.在基于LLG方程的宏自旋（Macrospin）模型中，引入平面内的退磁因子分量，重点研究了垂直磁各向异性（Perpendicular Magnetic Anisotropy，PMA）铁磁材料的磁隧道结（Magnetic Tunnel Junction，MTJ）中自由层磁矩翻转时间及阈值电流随钉扎层磁矩倾角的变化。研究结果表明，小的钉扎层磁矩倾角可以大幅改善磁矩的翻转时间与阈值电流密度。5.具体讨论了宏自旋（Macrospin）模型中器件形状对磁矩翻转特性的影响，给出了自由层磁矩翻转时间与阈值电流密度随自由层长度、厚度变化的特征。同时也分析了自由层磁矩倾角对磁矩翻转时间及阈值电流密度的影响，并与钉扎层磁矩倾角产生的结果进行了对比。结果表明，在垂直结构中，小的磁矩倾角及合适长厚比可以大幅度的缩短磁矩翻转时间及减小所需阈值电流。另外，与传统的算法相比，相同条件下所得到的磁矩翻转时间及阈值电流密度值都要较高，且更为准确。更多还原

【Abstract】 With advancement in information technology and the arrival of the big dataera, data resource has become one of the indispensable necessities of life.Meanwhile, the development of data storage devices that are of smaller size,larger capacity, and higher reliability has been unabated. Therefore, constantlyimproving the storage density and enhancing the performance of storage devicesare of great significance. The perpendicular spin transfer torque magneticrandom access memory (STT-MRAM) is one of the promising candidates for thenext generation memory devices due to its fast speed, non-volatile features andits nearly infinite number of write-read cycles, and indeed, its potential forachieving extremely high level of storage density as compared to the otheralternatives for solid-state data storage devices.In nanoscale magnetic structures, magnetization switching dynamics is adirect measure of the storage device performance. Thus, research into themagnetization dynamics is of great significance to magnetic storage technologyand spintronics. In the previous STT-MRAM devices, an in in-plane structurehas been used. In order to reduce the current density, the magnetic memory has to have a larger cross section. Therefore, in analysis of the magnetizationdynamics in such structure, a simplified macrospin model can be used, in thatonly the effect induced by demagnetization factor in the vertical direction needsto be taken into consideration, or the cross section is assumed to be infinitelylarge. However, due to relatively smaller switching current for the perpendicularstructure of MTJ, the cross-sectional area can be further reduced while retainingthe thermal stability of the device. In this case, the simplified macrospin modelappears to ineffective with the effect of cross-sectional area being neglected.Additionally, the recent research has shown that changing the free layer orpolarization layer thickness can result in a small tilt angle of the magneticmoment with respect to the vertical direction, as observed in the magneticstructures using CoFeB thin film or Co/Pd multilayer film, for example.However, the influence of such tilt angle on the magnetization under the finitesize of the cross-sectional area, remains to be studied. To this end, this papermainly studies the following aspects around dynamic features of magnetizationswitching:1. Chapter1introduces the significance and development history of themagnetic storage technology, along with the present status and futuredevelopment direction of such technology.2. Chapter2introduces the demagnetizing field and demagnetizing energy.With the point of the magnetic dipole interaction, the formula of demagnetizingfield is derived in detail. Finally, generalizations about the components of demagnetization factor of the ellipsoid and the rectangular structure are given.3. In Chapter3, it concretely analyzes the methods of dealing with themacro spin model, and summarizes its advantages and disadvantages, byutilizing numerical simulation method, especially focusing on the effect the spintransfer torque (STT) effect.4. Base on the Landau-Lifshitz-Gilbert (LLG) equation which hasintroduced the component of demagnetizing factor of in-plane structure, thechapter4mainly investigate the effects of magnetization tilt angle of pinnedlayer on the dynamic characteristics of the magnetization reversal in a magnetictunnel junction (MTJ) having a free layer with the perpendicular magneticanisotropy (PMA). The results demonstrates that the switching time and therequired threshold current density can be significantly reduced for aperpendicular MTJ structure when using a relatively small magnetization tiltangle of the pinned layer.5. Usingmacrospin model, the influences of the free layer’s shape and sizeson the magnetization reversal characteristics have been discussed. Thedependence of the threshold current density and the switching time as a functionof the ratio of the free layer thickness over its characteristic length has beenstudied. The influences of the magnetization tilt angle of free layer on thethreshold current density and the switching time have also been investigated,and compared with the result in which dynamic characteristics of themagnetization reversal is influenced by the effect of magnetization tilt angle of the pinned layer. The results demonstrates that the switching time and therequired threshold current density can be significantly reduced for aperpendicular MTJ structure when using a relatively small magnetization tiltangle of free layer and appropriate aspect ratio of the free layer. Moreover,comparing with conventional arithmetic, larger and more accurate switchingtime and threshold current density can be obtained under the same condition.更多还原