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Characterization of spin-orbit torques in Pt/Co/Ta structures

机译：Pt / Co / Ta结构中自旋轨道转矩的表征

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Current-induced torques in heavy-metal/ferromagnet/oxide stacks have been of significant recent interest for highly efficient magnetization switching and domain wall motion. These spin-orbit torques (SOTs) arise through the spin-hall effect (SHE) and Rashba effects at the heavy-metal/ ferromagnet interface. In such structures, the oxide layer plays the role of breaking the inversion symmetry of the structure, but typically does not actively contribute to the SOT. Here we examine SOTs in ultrathin Co films sandwiched between two spin Hall metals whose spin Hall angles are of opposite sign. In this case, the Slonczewski-like torques generated at the top in bottom interface work in concert to enhance the total SOT. We examine Pt/Co/Ta stacks, where effective spin Hall angles have been reported in the ranges θ=-0.12~0.15 and θ= +0.04~0.08 in torque measurements. A series of Pt(3nm)/Co(0.9nm)/Ta(t) tri-layer structure capped by 1.5nm of TaOx were prepared by sputter deposition, where the thickness t of the Ta metal top layer varied from t=0.5nm to t=4nm. These films all exhibited perpendicular magnetic anisotropy in the as-deposited state. As shown in the figure 1, SOTs were measured using the harmonic Hall voltage measurement scheme, in which the variation of the first and second harmonics of the anomalous Hall voltage with in-plane fields are used to quantify the longitudinal and transverse induced effective fields generated, respectively, by the Slonczewski-like and field-like SOTs. Figure 1(a) and (b) shows the experimental geometry for the torque measurements, and the first and second harmonics of the anomalous Hall voltage, V and V, are then measured while sweeping either a longitudinal field H or transverse field H to yield H, and H , respectively. Figure 1(g) shows the measured effective fields, H and H- sub>SL, depending on the thickness t of the Ta top metal layer (left axis). The Sloncewski-like torque increases substantially up to 190 Oe per 10 A/m, along with a sizable field-like torque that exceeds 120 Oe per 10 A/m with increasing Ta layer thickness. The effective spin Hall angle computed from the H is shown referenced to the right-hand axis of Fig 1(g). An effective spin Hall angle of up to 34% is observed, exceeding the record value of 0.30 for W. X-ray photoelectron spectroscopy (XPS) sputter-depth profiling was performed to extract the depth-dependent material compositions. Based on the XPS profiling results, we speculate that the presence of Ta within the Co layer and the compositionally-graded Co/Ta interface may increase asymmetric spin scattering within the Co layer and/or enhance the spin injection efficiency from the Ta to Co due to the diffuse nature of the interface. Finally, we characterized current-induced switching and extracted a measure of the switching efficiency to compare with the effective fields obtained from harmonic SOT measurements. Figure 2(a, b) shows exemplary switching phase diagrams for t=0.5nm and t=4nm in which the mean normalized M after current pulse injection was determined for each pair (H, j) from 10 measurement cycles. As shown in Figure 2(c), the switching efficiency increased significantly with the addition of a metallic Ta overlayer, by about a factor of 2 over the range of t examined, implying that the large enhancement in the Slonczewski-like torque significantly increased the current-induced switching efficiency. These results point to significant opportunities to engineer the interfaces of ultrathin transition ferromagnets to enhance SOTs for spintronic device applications.
机译：最近，重金属/铁磁体/氧化物堆中的电流感应转矩对于高效磁化切换和磁畴壁运动具有重大意义。这些自旋轨道转矩（SOT）通过重金属/铁磁体界面处的自旋霍尔效应（SHE）和拉什巴效应产生。在这样的结构中，氧化物层起到破坏结构的反转对称性的作用，但是通常不会对SOT产生积极的贡献。在这里，我们研究了夹在两种自旋霍尔金属之间的超薄Co膜中的SOT，这两种自旋霍尔金属的符号相反。在这种情况下，在底部界面的顶部产生的类似Slonczewski的扭矩协同工作，以提高总的SOT。我们检查了Pt / Co / Ta堆，其中在扭矩测量中已报告了有效自旋霍尔角在θ= -0.12〜0.15和θ= + 0.04〜0.08的范围内。通过溅射沉积制备了一系列被1.5nm TaOx覆盖的Pt（3nm）/ Co（0.9nm）/ Ta（t）三层结构，其中Ta金属顶层的厚度t从t = 0.5nm变化到t ＝ 4nm。这些膜在沉积状态下均表现出垂直磁各向异性。如图1所示，使用谐波霍尔电压测量方案测量SOT，其中使用异常霍尔电压的一次和二次谐波随面内场的变化来量化所产生的纵向和横向感应有效场分别由类似Slonczewski和类似SOT的SOT组成。图1（a）和（b）显示了扭矩测量的实验几何形状，然后在扫描纵向磁场H或横向磁场H的同时测量异常霍尔电压V和V的一次和二次谐波H和H分别。图1（g）显示了测量的有效场H和H-sub> SL，取决于Ta顶部金属层的厚度t（左轴）。随着Ta层厚度的增加，类似于Sloncewski的扭矩基本上增加到每10 A / m 190 Oe，随着超过10 O / m的相当大的类似磁场的扭矩超过10 O / m 10 A / m。参照图1（g）的右轴，显示了从H计算的有效自旋霍尔角。观察到高达34％的有效自旋霍尔角，超过W的记录值0.30。执行了X射线光电子能谱（XPS）溅射深度分析，以提取取决于深度的材料成分。根据XPS分析结果，我们推测在Co层中存在Ta以及成分渐变的Co / Ta界面可能会增加Co层中的不对称自旋散射和/或增强从Ta到Co的自旋注入效率界面的分散性质。最后，我们对电流感应开关进行了表征，并提取了开关效率的量度，以与从谐波SOT测量获得的有效场进行比较。图2（a，b）显示了t = 0.5nm和t = 4nm的示例性开关相位图，其中从10个测量周期中为每对（H，j）确定了电流脉冲注入后的平均归一化M。如图2（c）所示，在添加的金属Ta覆盖层的情况下，开关效率在t的测试范围内显着提高了约2倍，这意味着Slonczewski式转矩的大幅提高显着提高了T电流感应开关效率。这些结果表明，设计超薄过渡铁磁体的接口以增强自旋电子器件应用的SOT的重大机会。

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