Maximizing Spin-Orbit-Torque Efficiency of Pt/Ti Multilayers: Trade-Off Between Intrinsic Spin Hall Conductivity and Carrier Lifetime

Abstract

We report a comprehensive study of the maximization of the spin Hall ratio (θSH) in Pt thin films by the insertion of submonolayer layers of Ti to decrease carrier lifetime while minimizing the concurrent reduction in the spin Hall conductivity. We establish that the intrinsic spin Hall conductivity of Pt, while robust against the strain and the moderate interruption of crystal order caused by these insertions, begins to decrease rapidly at high resistivity level because of the shortening carrier lifetime. The unavoidable trade-off between the intrinsic spin Hall conductivity and carrier lifetime sets a practical upper bound of θSH ≥0.8 for heterogeneous materials where the crystalline Pt component is the source of the spin Hall effect and the resistivity is increased by shortening carrier lifetime. This work also establishes a very promising spin-Hall metal of [Pt 0.75 nm/Ti 0.2 nm]7/Pt 0.75 nm for energy-efficient, high-endurance spin-orbit torque technologies (e.g., memories, oscillators, and logic) due to its combination of a giant θSH ≈ 0.8, or equivalently a dampinglike spin torque efficiency per unit current density DL ≈ 0.35, with a relatively low resistivity (90 μΩ cm) and high suitability for practical technology integration.Spin Hall metals with strong dampinglike spin-orbit torque (SOT) efficiency per unit current density ( DL ) and relatively low resistivity (ρxx) are the key for developing practical spin-orbit torque technologies (e.g., memories, oscillators, and logic)[1-10] that require energy efficiency, high endurance, and low impedance [11,12]. For example, for a SOT-MRAM device with a spin Hall channel (with thickness dHM and resistivity ρxx)/FM free layer (with thickness t and resistivity ρFM), the write current is approximately Iwrite∝ (1+s) / DL , and the corresponding write energy is Pwrite ∝ [(1+s) / DL ] 2 ρxx, where s ≈ tρxx/dHMρFM is the ratio of the current shunting in the FM layer over the current flow in the spin Hall channel (see Fig. 1(a)). Meanwhile, a high ρxx of a spin Hall material (e.g., ρxx ≥ 200 μΩ cm in Ta [2], W [8,13,14], and topological insulators [15,16]) is problematic for applications that require a high endurance [17] and low device impedance (∝ρxx) [18]. For example, use of a large-ρxx spin Hall material (e.g., ρxx =200 -300 μΩ cm for W, see Fig. 1(b)) will limit the endurance of SOT devices via Joule-heating-induced bursting and migration of the write line [17] as well as result in a high write impedance that is difficult for superconducting circuits in a cryogenic computing system to accommodate [18]. It is therefore of great technological and fundamental importance to establish how, why, and to what limit the spin Hall ratio (θSH) and DL of a spin Hall metal with a giant spin Hall conductivity (σSH) and a relatively low ρxx can be enhanced in practice.Among the various spin Hall metals, Pt is particularly attractive for spin-torque technologies due to its low ρxx, the highest intrinsic σSH known for the simple technologically viable metals (> 1.6×10 6 (ℏ/2e) Ω ...

Origin of Strong Two-Magnon Scattering in Heavy-Metal/Ferromagnet/Oxide Heterostructures

“…It warrants future efforts to unveil the cause of the different values of DL E , but that is beyond the scope of this work. Here, it worth mentioning that the "in-plane" harmonic Hall response measurements, if performed carefully [26,27], yield results of DL E that are consistent with those obtained from "out-of-plane" harmonic Hall response measurements [44,45] and antidamping SOT switching of in-plane magnetized 3-terminal magnetic tunnel junctions [46,47].…”

“…Sample series A1-A6, B1-B3, and C1 (Table 1) were patterned into magnetic strips (10×20 μm 2 ) via photolithography and ion milling to measure α and the effective demagnetization field (4πM eff ) and the SOT efficiencies using spin-torque ferromagnetic resonance (ST-FMR) [24,25] and also into Hall bars (5 × 65 μm 2 ) for determination of SOT efficiencies using "in-plane" harmonic Hall response technique [26][27][28]. α and 4πM eff of sample series B4 and C2-C6 (Table 1) were obtained from unpatterned pieces using a flip-chip FMR with the magnetic pieces face-side down on a co-plane waveguide.…”

“…To examine the possible effects of the non-uniform short-wavelength magnons on the SOT exerted on the uniform mode, we determined the dampinglike SOT efficiency for the Co/Pt (A1), Pt/Co (A2) and Pt/Hf/Co (A3) using both ST-FMR measurements [24] and harmonic response measurements [26,27]. For the ST-FMR determination, if we employ the standard macrospin analysis and assume a negligible spin pumping effect, we can define an effective FMR spin-torque efficiency ξ FMR [24] from the ratio of the symmetric (S) and anti-symmetric (A) components of the magnetoresistance response of the ST-FMR resonance.…”

“…6(b), DL E was estimated from the ST-FMR measurement to be (0.98 ± 0.03)×10 5 Ω -1 m -1 for Co/Pt (A1), to (1.62 ± 0.04)×10 5 As described in more detail in Refs. [27,28], the cosφ…”

Giant spin torque efficiency in single-crystalline antiferromagnet Mn2Au films

“…In a spin-orbit torque (SOT) magnetic random-access mem-ory (SOT-MRAM), owing to the negligible stray magnetic fields and the resilience to external magnetic field perturbations, AFM materials can effectively suppress the crosstalk between stored information and thereafter help reduce the recording bit size [6]. In the recording bit of the SOT-MRAM device with a damping-like spin Hall channel/ferromagnetic (FM) free layer structure, it is of fundamental importance for the spin Hall channel materials to possess a high and controllable SOT efficiency ξ DL , because the writing current I write is proportional to 1/ξ DL [7]. Compared with the nonmagnetic (NM) metallic materials with ξ DL > 0.5, such as β-W and Pt-derived Pt/ Ti multilayer structure [7][8][9][10][11][12], a lot of studies show that the metallic AFM materials, such as PtMn, IrMn and IrMn 3 , can provide comparable or even larger ξ DL , as shown in Table 1 [13][14][15][16][17].…”