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具有强自旋轨道耦合(SOC)效应的稀土金属因其可以产生自旋霍尔矩有望在低功耗磁信息存储、逻辑运算和神经元模拟器件中发挥潜在作用. 本文选用重稀土金属镝(Dy)作为自旋源层, [Co/Pt]3作为磁性层构建Dy/Pt/[Co/Pt]3垂直磁化多层膜, 探究了不同Dy层厚度对体系自旋轨道矩(SOT)效率以及SOT驱动磁矩翻转的影响规律. 利用谐波锁相技术分析得到稀土金属Dy的内禀自旋霍尔角为0.260±0.039, 自旋扩散长度为(2.234±0.383) nm, 表明Dy可以作为理想的自旋源材料. 此外, 基于体系类阻尼SOT效率的有效提升, 临界翻转电流密度随Dy层厚度增加而逐渐降低, 最低约为5.3×106 A/cm2. 以上研究结果证实稀土金属Dy存在强的自旋霍尔效应, 为设计低功耗SOT基自旋电子器件提供了有效路径.Spin-orbit torque (SOT) based on the spin-orbit coupling (SOC) effect has received increasing attention in magnetic information storage, logical operation and neuron simulation devices because it can effectively manipulate magnetization conversion, chiral magnetic domain walls, and magnetic skyrmion motions. Further improvement of the SOT efficiency and reduction of the driving current density are crucial scientific problems to be solved for high-density and low-power applications of SOT-based spintronic devices. The heavy rare-earth metal dysprosium (Dy) possesses a relatively strong SOC due to the partially filled f orbital electrons (4f10), which is expected to generate spin Hall torques. In this work, the influences of Dy thickness on the SOT efficiency and SOT-driven magnetic reversal are explored in the Dy/Pt/[Co/Pt]3 magnetic multilayers, where the rare-earth Dy and [Co/Pt]3 are used as a spin-source layer and a perpendicularly magnetized ferromagnetic layer, respectively. A series of Dy/Pt/[Co/Pt]3 heterostructures with the values of Dy layer thickness (tDy) of 1, 3, 5 and 7 nm is fabricated by ultrahigh-vacuum magnetron sputtering. The perpendicular magnetic anisotropy, SOT efficiency, spin Hall angle and current-induced magnetization switching are characterized using the magnetic property and electrical transport measurements. The results show that the conversion field and magnetic anisotropic field decrease with the increase of tDy, revealing that the magnetic parameters can be regulated by the bottom Dy layer due to their structural sensitivity. However, both damping-like SOT efficiency and effective spin Hall angle (θeff SH) gradually increase with the increase of tDy, indicating that the rare-earth Dy can provide additional spin current to enhance the SOT efficiency apart from the contribution of Pt/[Co/Pt]3. Particularly, the maximum value of θeff SHof 0.379±0.008 is achieved when tDy is 7 nm. According to the fitting analysis of the drift-diffusion model, the intrinsic spin Hall angle and spin diffusion length of the rare-earth Dy are extracted to be 0.260±0.039 and (2.234±0.383) nm, respectively, suggesting that Dy can be used as an ideal spin-source material. In addition, the critical conversion current density (Jc) gradually decreases with the increase of tDy, and Jc reaches a minimum value of approximately 5.3×106 A/cm2 at tDy = 7 nm, which is mainly attributed to the increase of the damping-like SOT and slight decrease of the switching field. These results experimentally demonstrate a strong spin Hall effect of the rare-earth Dy, and provide a feasible route for designing SOT-based spintronic devices with low-power dissipation.
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图 1 (a) Dy/Pt/[Co/Pt]3多层膜的结构示意图; (b)霍尔器件的光学显微图像和电输运测量示意图; (c)不同Dy厚度样品的反常霍尔曲线, 测试电流为1 mA, 插图为t = 7 nm样品的面内磁滞回线; (d)饱和磁化强度、翻转场和磁各向异性常数随Dy层厚度的变化关系
Fig. 1. (a) Schematic diagram of a Dy/Pt/[Co/Pt]3 stack; (b) optical image of the Hall device accompanied by a schematic measurement setup; (c) anomalous Hall loops for stacks with varying Dy layer thicknesses measured at I = 1 mA, and the inset presents the in-plane magnetic hysteresis loop for the stack with t = 7 nm; (d) dependence of Ms, Hsw, and Ku on Dy layer thickness.
图 2 类阻尼(a)和类场(b)有效场的测量示意图; 一阶谐波电压(Vω)和二阶谐波电压(V2ω)随面内纵向扫描磁场HL(c)和横向扫描磁场HT(d)的变化关系
Fig. 2. Schematic measurement setups of damping-like (a) and field-like (b) effective fields; representative first (Vω) and second (V2ω) harmonic voltages as functions of in-plane longitudinal magnetic field HL (c) and transverse magnetic field HT (d).
图 3 (a)平面霍尔电阻随方位角ϕ的变化关系; 平面霍尔效应修正的类阻尼有效场(b)和类场有效场(c)随正弦电流幅值I0的依赖关系
Fig. 3. (a) Dependence of the planar Hall resistance on the azimuthal angle ϕ for stacks with different Dy layer thicknesses; Calibrated damping-like (b) and field-like (c) effective fields against the amplitude of the input sinusoidal current by considering the planar Hall effect.
图 4 类阻尼SOT效率(a)、类场SOT效率(b)和有效自旋霍尔角(c)随Dy层厚度的变化关系
Fig. 4. Damping-like SOT efficiency (a), field-like SOT efficiency (b), and effective spin Hall angle (c) as a function of Dy layer thickness.
图 5 (a) t = 7 nm样品在不同面内辅助场Hx的电流驱动磁矩翻转回线; (b)不同Dy层厚度样品在Hx = +600 Oe的电流驱动磁矩翻转回线; (c)不同Dy层厚度样品的电流驱动磁矩翻转相图; (d) Hx = +600 Oe时Dy/Pt/[Co/Pt]3, Pt/[Co/Pt]3和Dy层中临界翻转电流密度以及翻转效率随Dy层厚度的变化关系
Fig. 5. (a) Current-driven magnetization switching loops for the stack with t = 7 nm under various in-plane bias magnetic fields; (b) current-driven magnetization switching loops for stacks with different Dy layer thicknesses under Hx = +600 Oe; (c) switching phase diagram for stacks with different Dy layer thicknesses; (d) dependence of the critical switching current density in the Dy/Pt/[Co/Pt]3, Pt/[Co/Pt]3 and Dy layers as well as the switching efficiency on Dy layer thickness under Hx = +600 Oe.
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