1.Institute of Advanced Materials, Beijing Normal University, Beijing 100875, China 2.Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 3.Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Hong Kong 999077, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 51901025, 60573172, 51625101), the Key Program of the Natural Science Foundation of Beijing, China (Grant Nos. Z190007, Z190009), the Fundamental Research Funds for the Central Universities, China (Grant No. 310421101), and the Hong Kong Research Grants Council, China (Grant No. ECS26200520)
Received Date:02 January 2021
Accepted Date:24 January 2021
Available Online:24 February 2021
Published Online:20 June 2021
Abstract:The spin-orbit torque generated by charge current in a strong spin-orbit coupling material provides a fast and efficient way to manipulate the magnetic moment in adjacent magnetic layers, which is expected to be used for developing low-power, high-performance spintronic devices. Two-dimensional materials have attracted great attention, for example, they have abundant species, a variety of crystal structures and symmetries, good adjustability of spin-orbit coupling strength and conductivity, and good ability to overcome the lattice mismatch to form high-quality heterojunctions, thereby providing a unique platform for studying the spin-orbit torques. This paper covers the latest research progress of spin-orbital torques in two-dimensional materials and their heterostructures, including their generations, characteristics, and magnetization manipulations in the heterostructures based on non-magnetic two-dimensional materials (such as MoS2, WSe2, WS2, WTe2, TaTe2, MoTe2, NbSe2, PtTe2, TaS2, etc.) and magnetic two-dimensional materials (such as Fe3GeTe2, Cr2Ge2Te6, etc.). Finally, some problems remaining to be solved and challenges are pointed out, and the possible research directions and potential applications of two-dimensional material spin-orbit torque are also proposed. Keywords:two-dimensional materials/ spin-orbit coupling/ spin-orbit torque/ current-driven magnetization switching
表1已报道的实验研究工作中TMD材料的晶体结构、制备方法、TMD/FM异质结中的SOT的表征方法以及自旋霍尔电导 Table1.Crystal structure, preparation method, method for SOT measurement of the TMD/FM heterostructure, and spin Hall conductance of TMD materials in the previous studies.
22.1.二维半导体材料
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2.1.二维半导体材料
2H-MoS2是一种二维半导体材料, 因具有宽能隙和高迁移率等特性而备受关注. 它具有六角晶格结构, 每个Mo原子与6个最近邻S原子结合, 属于P6/mmc空间群. 单层MoS2与FM层接触时, 由于界面对称性的破缺, 电流能够诱导产生类阻尼SOT$ ({\tau }_{\rm{DL}}) $和类场SOT($ {\tau }_{\rm{FL}} $)两种效应. Zhang等[59]最早通过ST-FMR技术研究了MoS2/Ni80Fe20(Py)异质结中的SOT. 其中MoS2是通过CVD方法制备的单层三角形晶粒. 如图1(a)和图1(b)所示, 其对称峰值(与类阻尼SOT相关)约为反对称峰值(与类场SOT相关)的4倍, 表明类阻尼SOT可能比类场SOT大得多. 由于逆Rashba-Edelstein效应诱导的自旋泵浦可能也有很大的贡献, 该工作没有量化SOT的强度. Shao等[60]进一步通过二次谐波霍尔方法定量研究了MoS2(WSe2)/CoFeB异质结中电流产生的SOT. 其中, MoS2是CVD方法生长的大面积单层薄膜. 如图1(c)所示. 测量得到的二次谐波霍尔电阻$ {R}_{\rm{H}}^{2\omega }\left(\varphi \right) $的典型方位角依赖性如图1(d)所示. 通过分析二次谐波霍尔电阻对方位角(φ)依赖性的不同, 可以分离得到类阻尼SOT和类场SOT的大小. 实验结果表明, MoS2, WSe2的类场SOT对应的自旋电导分别为2.9 × 103${\hbar /(2{\rm{e}})}$ (Ω·m)–1, 5.5 × 103${\hbar /(2{\rm{e}})}$ (Ω·m)–1; 类阻尼SOT在实验测试误差范围内为零. Shao等[60]认为, 实验到观察到的类场SOT主要归因于界面Rashba-Edelstein效应. 当前, 大多数的SOT器件采用外场辅助下类阻尼SOT驱动垂直磁矩翻转的操控机制, 因此获得大的类阻尼SOT是人们所期望的. 虽然上述工作表明CVD生长的单层MoS2和WSe2可能无法产生人们所需要的较大的类阻尼SOT, 但较大的类场SOT的发现让人们看到了利用TMD材料产生SOT的信心, 因此激发了更多的科研人员开展TMD材料中SOT的相关研究. 图 1 MoS2/Py异质结中ST-FMR信号的对称(a)和反对称(b)振幅随外加磁场与平面夹角θ的依赖关系(插图为基于MoS2/Py异质结的ST-FMR器件光学显微镜图)[59]; (c) MX2/CoFeB 异质结的SOT测量装置示意图; (d) 二次谐波方法测得二阶霍尔电阻与φ的函数关系, 外加磁场为100 Oe (1 Oe = 103/(4π) A/m)[60]; (e) WS2/Py双层器件几何结构示意图, 其中Vg通过SiO2介质层施加; (f)Vg对Py和WS2/Py双层的转矩比$ {\tau }_{\rm{FL}}/{\tau }_{\rm{DL}} $调控特性[61] Figure1. Out-of-plane (OOP) angular (the applied field is described by the polar angle) dependence of symmetric (a) and antisymmetric (b) components of the ST-FMR signal based on MoS2/Py heterostructure (the inset is photo image of ST-FMR device)[59]; (c) measurement setup of SOT measurements for the MX2/CoFeB bilayer; (d) second-harmonic Hall resistance as a function of φ with an external magnetic field 100 Oe applied[60]; (e) schematic of the WS2/Py bilayer device geometry, where Vg was applied through the SiO2 dielectric layer; (f) torque ratio $ {\tau }_{\rm{FL}}/{\tau }_{\rm{DL}} $ dependence of Vg for Py and WS2/Py bilayer[61].
除了半导体TMD材料之外, (半)金属TMD材料也被广泛地研究. 研究这类材料的主要驱动力在于它们具有高导电性、强自旋-轨道耦合、低结构对称性等. 人们最早研究的(半)金属TMD材料是WTe2, 属于Pmn21空间群. 同MoS2相比, 它具有更低的对称性, 满足产生非传统SOT的对称性要求. MacNeill等[58]首次利用ST-FMR技术研究了WTe2/Py异质结构中的SOT, 如图2(a)所示. 除了传统的类场SOT和类阻尼SOT, 实验还观察到了非传统的面外SOT (图2(b)), 其对应的自旋霍尔电导为$\left( {3.6 \pm 0.8} \right) \times {10^3} {\hbar /(2{\rm{e}})} \;{\left( {{{\Omega }}\cdot{\rm{m}}} \right)^{ - 1}}$, 且大小随着施加电流和WTe2晶体a轴的角度的增大而减小. 当施加电流和a轴垂直, 即沿b轴时, 非传统SOT消失, 这表明了非传统SOT与晶体的对称性有关. 尽管早期的研究中显示SOT对厚度的依赖关系很小, 随着更深入的研究及更宽的厚度范围的研究, 传统和非传统SOT都表现出了厚度的依懒性, 说明其微观起源除了界面效应还有体效应的贡献[62,67]. Shi等[62]在WTe2/Py异质结构中实现了非常有效的电流驱动平面内磁矩翻转(图2(c)), 翻转电流密度约为2.96 × 105 A/cm2. 更有趣的是, 在该体系中, 还观察到了Dzyaloshinskii-Moriya相互作用, 为进一步研究手性磁结构提供了材料基础. 图 2 (a) WTe2/Py异质结样品几何结构示意图; (b) WTe2/Py器件的对称和反对称ST-FMR信号与面内磁场角度的依赖关系, 其中电流平行于a轴[58]; (c) 由MOKE图像捕捉到的电流驱动磁矩翻转过程[62]; (d) 自旋电导率随MoTe2厚度的变化关系[65]; (e) MoTe2单斜1T′相的晶体结构和20层MoTe2薄膜的能带结构[70]; (f) PtTe2/Py器件ST-FMR测量SOT效率ξSOT和自旋霍尔电导率$ {\sigma }_{\rm{s}} $的厚度依赖性; (g) PtTe2/Au/CoTb结构和PtTe2中电流产生的SOT的示意图; (h)在不同的面内磁场下, PtTe2中电流产生的SOT驱动具有垂直磁各向异性的CoTb层磁矩翻转[68] Figure2. (a) Schematic of the bilayer WTe2/Py sample geometry; (b) symmetric and antisymmetric ST-FMR resonance components for a WTe2 (5.5 nm)/Py (6 nm) device as a function of in-plane magnetic-field angle, with current applied parallel to the a-axis[58]; (c) switching process captured by MOKE images[62]; (d) spin conductivities as a function of the thickness of MoTe2, where σS stands for the conventional damping-like torque, σB stands for the out-of-plane damping-like torque, and σT stands for the out-of-plane field-like torque[65]; (e) crystal structure of the monoclinic 1T′ phase of MoTe2 and band structure of a MoTe2 slab with 20 monolayers[70]; (f) thickness dependence of ξSOT and spin Hall conductivity σs of PtTe2/Py measured by ST-FMR; (g) schematic layout for PtTe2/Au/CoTb stack and the SOT generated by the majority of current flowing in PtTe2; (h) current-induced switching of the CoTb layer by SOT from PtTe2 under different in-plane field[68].