自旋轨道转矩效应驱动的磁动力学研究

Magnetic random access memories (MRAMs) utilizing perpendicular current-induced spin-transfer torques (STTs) have been commercialized since 2012, however, they still have some unsolved issues which restrict the practical applications. Recent studies demonstrate that an in-plane flowing current can induce spin-orbit torques (SOTs), which provides an alternative and efficient route for the manipulation of magnetization and can be used to develop the new three-terminal SOT-MRAMs. In contrast to STTs, the generation of SOTs is independent of the ferromagnetic reference layer in magnetic tunnel junctions. In this case the SOT torque is orthogonal to the quiescent free layer magnetization, leading to a negligible incubation delay and hence fast switching speed. Another crucial feature is the separation of the read and write current paths in the SOT devices, which avoids electrical stress of the oxide barrier during writing. Based on these advantages, the study of SOT-MRAMs has attracted a great deal of interest. In this project, we will perform a detailed study regarding this important scientific area by using micromagnetic simulations and time-resolved MOKE techniques. We will investigate the influence of SOT effect on the magnetic dynamic parameters and gain a deep insight into the underlying physical mechanism and characteristics of SOT-driven magnetization switching. We will clarify how and why the SOT effect drives magnetization switching, either by coherent rotation or by domain nucleation and propagation. Furthermore, field-free SOT switching will be explored and the optimal SOT device structure will be designed. We expect these studies shall provide some new insights into better understanding of the SOT-driven magnetization dynamics for SOT-MRAM developments.

垂直电流直写自旋转矩型STT-MRAM已经产品化，但仍存在一些制约因素。最近发现，基于面内电流诱导的“自旋轨道转矩（SOT）效应”可用于发展另一种新型电流驱动磁存储器SOT-MRAM。与STT不同，SOT转矩的产生与参考层磁化方向无关，翻转速度快，无需孕育期。同时，其三端式存储器设计使得信号写入和读取路径分开，有效避免了电流写操作对磁性隧道结氧化层的破坏，提高器件寿命。因此，SOT-MRAM引起人们的极大研究兴趣，目前尚处于起步研究阶段。本项目我们将针对这一前沿科学问题开展微磁学和TR-MOKE相结合的研究，探讨面内电流的SOT效应对磁超快动学参数的影响，阐明SOT驱动磁化翻转的物理机制和行为特性；揭示一致进动翻转或是畴壁移动翻转的微观机理及其依赖关系；研究无外磁场辅助的纯SOT驱动磁化翻转优化结构等。我们期望这些研究有助于加深SOT驱动磁动力学的理解，推动SOT-MRAM的发展。

为加深SOT驱动磁化翻转动力学的理解，进一步推动SOT-MRAM的发展，本项目针对自旋流的产生、无磁场辅助下SOT翻转的优化结构和内在物理机制、SOT效应对磁动力学参数的影响和对磁化矢量的有效调控开展了系列的实验和理论研究。通过该项目的研究工作，我们不仅深入理解了电荷-自旋流转化和基于自旋霍尔效应产生的自旋轨道力矩（SHE-SOT）驱动磁化翻转的物理机制，提高了效率，掌握了提高自旋轨道矩效率的优化结构和有效途径，实现了无磁场辅助的确定性电流驱动磁化翻转。而且，我们还阐明了多种材料体系的超快磁动力学特性、影响磁阻尼因子的内外因素和各自的温度依赖性特征，确立了SAF体系中存在的多种磁矩共振模式、不同模式对应的偏置场条件以及频率变化规律。这些关于自旋电子学材料优化和物性调控的研究成果，不仅具有重要的科学意义，更是为研发高速、低能耗磁存储或逻辑运算器件提供必要的技术参数和参考依据。在该项目资助下，共发表研究论文33篇（其中本人为通讯作者的有29篇），授权国家发明专利1项，申请国家发明专利1项，培养博士生5人，硕士生2人，多次受邀做学术交流报告。