磁性异质结构薄膜自旋霍尔效应的研究

The spin current can be generated and detected electrically via direct/inverse spin Hall effect (SHE) in magnetic heterostructures, it plays a crucial role in the development of spintronics, especially in the emergent spin caloritronics and magnon spintronics. In this project, the heterostructures of “magnetic insulator/metal or alloy (nonmagnetic or ferromagnetic or antiferromagnetic)” and those with an interfacial inserting layer will be explored through spin Seebeck effect, spin pumping, and ferromagnetic resonance as well as other spin dependent transportation measurements. The SHE for different nonmagnetic binary alloys with controlled spin-orbit interaction magnitude, atomic disorder scattering and s-d scattering will be investigated, and their different contributions will be clarified. The large SHE of the narrow gap semiconducting metallic alloys without a heavy element will be thoroughly explored and the corresponding mechanism will be revealed. The metallic alloys with optimal extrinsic scattering and band structure will be explored to realize extremely enhanced SHE. The effect of the ferromagnetic and antiferromagnetic ordering as well as magnetic transition on the SHE of the ferromagnets and antiferromagnets will be investigated. Besides, the SHE anisotropy with respect to the magnetization vector direction and crystal axis will be studied to comprehensively reveal the anisotropic nature of the spin current transportation in ferromagnets and antiferromagnets. Finally spin transport efficiency in heterostructures with an interfacial insertion layer will be explored. The multi-reflection of electrons and quantum well effect within the insertion layer will be emphasized to significantly improve the spin transport efficiency.

磁性异质结构的“正/逆”自旋霍尔效应是纯电学方式“激化/探测”自旋流、调控自旋状态的核心，对未来自旋电子学，尤其对新发展的自旋热电子学、自旋波电子学非常关键。本项目以“磁性绝缘体/金属合金”双层异质结构以及带有“界面插层”的多层异质结构为具体对象，通过对异质结构的自旋赛贝克效应、自旋泵浦、铁磁共振及其它自旋相关输运性质的测量，研究金属合金的自旋-轨道耦合、原子无序散射和d带电子散射三者对自旋霍尔效应的影响和规律，揭示具有窄能隙半导体电子结构的金属合金的大自旋霍尔效应的新特点及其物理来源，发展新型大自旋霍尔角金属合金材料；研究不同磁有序状态、磁相变过程对逆自旋霍尔效应的影响与机制，发掘并深入了解磁有序材料的逆自旋霍尔效应的“磁致”各向异性和“晶体对称”各向异性；研究“界面插层”的电子多重反射和量子阱效应对异质结构界面自旋电导和自旋流透射效率的影响规律，发展高效的自旋流增透多层异质结构。

高效自旋流效应的“磁性绝缘体或金属/非磁金属”异质结构是自旋电子学的基本结构单元。我们研究了磁性绝缘体YIG薄膜的溅射生长，获得高质量外延YIG薄膜，相应YIG/Pt异质结构具有优异的自旋输运性质；发现经中等温度真空退火的YIG薄膜，其结构和宏观磁性变化很小，但导致少量缺陷对YIG/Pt的自旋输运性质产生多重影响。另外，获得具有垂直磁各向异性可调的高质量的溅射外延YIG纳米薄膜，相关YIG/Pt具有高效的界面自旋交换作用。.外延生长了垂直磁化HoIG薄膜，其饱和磁化强度和零磁矩补偿温度与块体材料相一致，有效垂直各向异性场达3.5 kOe，远大于垂直YIG的对应值；HoIG/Pt输运测量显示，磁近邻效应对反常霍尔效应的贡献占主导，但对自旋霍尔磁电阻和自旋塞贝克效应的结果影响不大；实现了电流的自旋-轨道力矩驱动垂直磁化HoIG/Pt磁矩翻转。 .发现bcc等原子比W-Cr合金由于强烈的原子无序散射，具有比β-W更大的自旋霍尔角，获得了一种具有大自旋霍尔角的结构稳定合金；同时，在YIG/W界面处观察到内建电场，该电场通过类似Rashba效应产生了额外的界面自旋-轨道耦合，降低了自旋流在界面的有效穿透率，证明通过界面电场可调控界面自旋混合电导。.设计制备了YIG/Cu/NiFe/IrMn自旋阀异质结构，通过自旋塞贝克效应测量，发现NiFe磁矩方向相对于自旋流极化方向的改变，逆自旋霍尔效应显著改变，极化方向垂直于NiFe磁矩与两者处于共线状态的有效自旋霍尔角之比达1.5，对应的自旋扩散长度分别为1.0 nm和2.8 nm。对Fe、Co、Ni系统研究，发现逆自旋霍尔效应由与金属磁化方向无关和强烈相关的两项构成，前者局限于界面附近，后者主要与金属磁性的体贡献相关联。.飞秒激光从磁性层激发超快自旋流并注入相邻非磁金属层，通过逆自旋霍尔效应转变为超快电流，可产生太赫兹波。以具有正、负大自旋霍尔角的Pt、W作为非磁层，以优化的金属三层结构W/Fe/Pt为基础，利用激光在[W/Fe/Pt/SiO2]n多周期结构的多重散射和干涉，抑制金属层对激光的反射和透射，提高激光的吸收率，增加太赫兹波的产生效率，在[W/Fe/Pt/SiO2]2中获得太赫兹波幅度1.7倍的增强。