垂直磁化膜中电流诱导自旋轨道力矩的研究

Spintronics has had significant impact on information technology. A crucial component of this next generation of spintronic applications is the control of magnetic orientation with an electrical current. The approach is furthest developed in magnetic tunnel junctions, which utilize spin transfer torque to reversibly switch the magnetization in one of the layers. An alternative approach has been demonstrated in recent experiments on bilayer systems consisting of ultrathin ferromagnetic metal (FM) layers adjacent to heavy metals (HMs). In these systems, spin-orbit coupling is responsible for current-induced torques, which are termed spin orbit torques (SOT) and are to be distinguished from conventional spin transfer torque since the system requires strong spin orbit coupling to generate the spins. The spin Hall effect and the current-induced spin polarization (i.e., the Rashba effect) are considered to be the major sources that enable spin current generation and spin accumulation, respectively, in ultrathin magnetic heterostructures. To date, it is thus essential to understand the origin of spin orbit torques in order to apply them for possible applications in spintronic devices. In this proposal, we study the spin orbit torque in CoFeB/MgO heterostructures with HM-based underlayers as a function of the structure of interfaces to provide insight into spin transport and action of nonequilibrium spins on magnetization. We will study the perpendicular magnetic anisotropy, which strongly depends on the structure of interfaces in HM/CoFeB/MgO. The SOT possesses a damping like torque and a field like torque, and the size and direction of both components of the torque vary depending on the materials and the layers' structure and can be investigated by means of the harmonic anomalous Hall effect and planar Hall effect. And then the action of spins on the magnetic moments will be evaluated by measuring the effective magnetic field, which reflects the size and direction of the spin orbit torque. We hope that the study of HM/CoFeB/MgO thin films tunned by the control of interfaces can offer a possible route for the applications of spintronics.

近年来，非磁重金属(HM)自旋霍尔效应的发现为利用自旋轨道相互作用来调控铁磁层磁化状态提供了可能。在非磁重金属/铁磁/氧化物这样的垂直磁化膜中，流经HM层中的电流作用于与之相邻近的铁磁层磁矩产生自旋轨道矩(SOT)，当电流的密度达到一个临界值时，SOT将会导致该铁磁层的磁矩发生翻转，但是目前SOT产生的基本物理机制还并不清楚，存在是界面Rashba效应还是自旋霍尔效应的争论。本申请从磁控溅射制备的CoFeB基新型垂直磁化膜的界面调控出发，分析HM/CoFeB/MgO 垂直磁化膜的磁各向异性及自旋相关的输运特性随界面掺杂元素或原位界面修饰的变化关系。着重于反常Hall效应和平面Hall效应的高次谐波测量手段，对电流诱导SOT驱动磁化翻转与HM/CoFeB/MgO 界面特性的关系进行系统的研究，揭示电流诱导SOT的物理机制，为电流诱导SOT在未来的自旋电子器件中的应用奠定基础。

本项目围绕钨基重金属垂直磁化膜的实现和自旋轨道力矩(SOT)的研究，设计并实现了多种新的垂直磁化膜材料，通过开展反常霍尔效应、高次谐波、以及电流诱导SOT驱动磁化翻转等测量，在SOT的物理机制研究中取得了新进展，获得了一批重要及首创的研究成果，达到了基金申请时提出的研究目标。项目取得的重要结果包括：(1) 通过对Zr/CoFeB/MgO这一新的垂直磁化膜SOT物理机制的研究，首次从实验上证实了轨道霍尔效应对SOT的贡献，弱自旋轨道耦合材料同样可以产生电流诱导SOT驱动磁化翻转，这一原创新的成果大大拓展了SOT的研究内容，结果发表在Phys. Rev. Research上。(2) 发现了目前国际上热稳定最高的用于SOT的垂直磁化膜，W/Zr/CoFeB/MgO，其最高热处理温度可达510 C，远远超过了之前的报道(430 C)，意味着该SOT器件可以与成熟的CMOS流程相结合，具有非常好的应用前景。(3) 结合申请人在中国散裂中子源上主持建造的极化中子反射谱仪，将极化中子反射技术(PNR)用于相关材料的薄膜磁性研究中，发表了采用国内自主研发的PNR技术开展研究的首篇研究论文，其重要性还在于验证了我国自主研发的PNR技术的可靠性，为该谱仪如期对用户开放奠定了基础。(4) 通过提供优质的垂直磁化膜给国内其他课题组，合作将SOT首次应用到逻辑电路中，这表明SOT可以拓展出新的器件应用领域，相关结果发表在Adv. Mater.。