铁磁与顺磁金属中的纯自旋流输运研究

Spintronic devices manipulating pure spin currents, flows of spin angular momentum without charge current, should play an important role in low energy consumption electronics of the next generation. Therefore, the methods for producing and manipulating pure spin current have recently attracted much attention. The Spin Hall Angle (SHA), which is given by the ratio of spin hall and charge conductivity, is a quantified parameter to describe the effectiveness of this spin-charge conversion. The usual methods to determine SHA, however, have critical defects, so that the experimentally reported values are quite different for nominally identical materials, even for similar methods utilizing spin pumping. In such a situation, the correct measurement of the SHA has been the most fundamental issue for studying spin current and the relevant materials. What we propose here, is to establish a new method to fix this critical problem. By using CoZr magnetic film to pump spin current into the nonmagnetic metal layer, the measured longitudinal voltage would be the pure inverse spin hall effect due to spin pumping. The reason is caused by the fact that the CoZr show quite weak anisotropic magnetoresistance, but with moderate anomalous hall effect, which can be used for determining the amplitude of the pure spin current when FMR happens. By doing that, we can build a correct measurement of the SHA. Moreover, we will systematically study the spin hall angle in 5d transition metals based materials. We will try to obtain the specific composition and condition for those materials with SHA as large as possible, and to find the reasonable approaches to further enhance spin hall effect in metals.

自旋霍尔效应是实现自旋流产生与探测的理论依据。由于自旋霍尔角是描述材料内部自旋流与电荷流之间转化效率的核心参数，所以获得具有大自旋霍尔角的材料是实现自旋流技术功能化的基础。但是这一关键参数到目前仍无法准确测量。本项目拟采用非线性自旋动力学原理结合独特的器件设计，首先解决自旋霍尔角准确测量这一关键科学问题；其次按照本征（单质）与非本征（杂质、缺陷和表面处的散射）自旋霍尔效应起源为理论基础，以人为控制非本征因素作为增大自旋霍尔角的主要技术途径，在自身自旋轨道较强的5d过渡族金属元素中寻找高自旋霍尔角的单质或合金体系。目标是1）建立自旋霍尔角的准确测量原理与技术，2）寻找自旋霍尔角大于0.1的材料体系。该研究不仅可以建立自旋霍尔角的准确测试方法，为相关材料研究提供技术保障，而且在获得高自旋霍尔效应材料体系的同时掌握这一关键参数的调控技术，从材料研究的角度推动自旋流技术在实际应用上的发展。

依据自旋流对自旋动力学的显著影响，利用自旋霍尔效应实现电荷流与自旋流的相互转化是发展高效率低能耗自旋电子学器件的有效途径。因此，自旋霍尔效应的定量表征与微观机理，既是实现高效率自旋流产生与检测的理论基础，又是优化材料性能推动自旋电子学器件发展的关键。本项目基本按照任务书中的研究计划执行，在近5年内分别开展了金属体系中自旋霍尔角的测量方法、几种典型金属与合金中的自旋霍尔角表征、自旋霍尔效应的微观机制等三个主要方向的研究工作，完成了立项时提出的研究内容，实现了项目的总体研究目标（建立金属材料自旋霍尔角准确测量的实验方法，获得自旋霍尔角大于0.1的金属材料）。我们将自旋动力学表征方法的优势推广到自旋霍尔效应研究，在自旋流产生、注入与检测的一体化器件中，1）利用自旋进动的非线性效应（表征自旋流强度）与自旋霍尔的空间对称性（表征电荷流强度），提出了具有局域特性的自旋霍尔效应检测方法，并有效剥离了自旋霍尔磁电阻对测量结果的干扰；2）在PtPd合金中获得了0.18的自旋霍尔角数值，同时研究了PtPd合金中决定器件性能的自旋流关键物理量（自旋霍尔角、自旋扩散长度、自旋流注入效率）与自旋霍尔效应物理本质-自旋轨道耦合的定量化关系，发现斜散射是增大自旋霍尔效应强度的主要微观机制。相关研究成果目前已发表8篇SCI论文，全部为项目负责人为第一作者或通讯联系作者，并表明资助。其中包括Physical Review B 1篇，Advanced Electronic Materials 1篇，Applied Physics Letters 3篇。在项目执行期内，已培养硕士毕业生4名，博士毕业生1名。