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

Abstract

Current-induced torques in heavy-metal/ferromagnet/oxide stacks have been of significant recent interest for highly efficient magnetization switching and domain wall motion. [1-5]. These spin-orbit torques (SOTs) arise through the spin-hall effect (SHE)[3-5] and Rashba effects at the heavy-metal/ ferromagnet interface [1]. 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 Ta =−0.12∼0.15 and θ Pt = +0.04∼0.08 [3-6] 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[7-8], 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 ω and V ω , are then measured while sweeping either a longitudinal field H L or transverse field H T to yield H SL , and H FL , respectively. Figure 1(g) shows the measured effective fields, H FL and H SL , depending on the thickness t Ta of the Ta top metal layer (left axis). The Sloncewski-like torque increases substantially up to 190 Oe per 1011 A/m2, along with a sizable field-like torque that exceeds 120 Oe per 1011 A/m2 with increasing Ta layer thickness. The effective spin Hall angle computed from the H SL 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[9]. 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 Ta =0.5nm and t Ta =4nm in which the mean normalized M z after current pulse injection was determined for each pair (H x , j p ulse ) 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 Ta 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.