Toward 100% Spin–Orbit Torque Efficiency with High Spin–Orbital Hall Conductivity Pt–Cr Alloys

Abstract

5d transition metal Pt is the canonical spin Hall material for efficient generation of spin−orbit torques (SOTs) in Pt/ferromagnetic layer (FM) heterostructures. However, for a long while with tremendous engineering endeavors, the damping-like SOT efficiencies (ξ DL ) of Pt and Pt alloys have still been limited to ξ DL < 0.5. Here we present that with proper alloying elements, particularly 3d transition metals V and Cr, a high spin−orbital Hall conductivity (σ SH ≈ 6.5 × 10 5 (ℏ/2e) Ω −1 m −1 ) can be developed. Especially for the Cr-doped case, an extremely high ξ DL ≈ 0.9 in a Pt 0.69 Cr 0.31 / Co device can be achieved with a moderate Pt 0.69 Cr 0.31 resistivity of ρ xx ≈ 133 μΩ cm. A low critical SOT-driven switching current density of J c ≈ 3.2 × 10 6 A cm −2 is also demonstrated. The damping constant (α) of the Pt 0.69 Cr 0.31 /FM structure is also found to be reduced to 0.052 from the pure Pt/FM case of 0.078. The overall high σ SH , giant ξ DL , moderate ρ xx , and reduced α of such a Pt−Cr/FM heterostructure makes it promising for versatile extremely low power consumption SOT memory applications.

Perpendicular Magnetization Switching Driven by Spin‐Orbit Torque for Artificial Synapses in Epitaxial Pt‐Based Multilayers

“…[ 34 ] The increased resistivity can also be attributed to the additional electronic scattering contribution by the interfacial Cr alloying after the annealing process. The corresponding enhancement of SOT efficiency was also reported in the Pt‐Cr/FM heterostructure and single layer L1 0 ‐FeCrPt, [ 35,36 ] but there is only a slight influence on this work due to the thick Pt thickness. Based on the above discussion, it is reasonable to conclude that the difference in SOT efficiencies between Pt (111)/Py and Pt (001)/Py is mainly attributed to the intrinsic facet dependence of SHE in the Pt sample.…”

Interfacial Engineering Strategies for Efficient Spin–Orbit Torque Devices with Pt Alloys

“…The resistivities (ρ xx ) of the deposited films are first characterized by four-point measurements at room temperature. As shown in Figure 1b, ρ xx values of the as-deposited Pt 1−x Cr x are in agreement with the results in our previous work: 32 37.2 μΩ•cm for pure Pt, 90.3 μΩ•cm for Pt 0.79 Cr 0.21 , and 117.7 μΩ•cm for Pt 0.70 Cr 0.30 . The resistivities after 400 °C annealing become 24.2 μΩ•cm for pure Pt, 129.7 μΩ•cm for Pt 0.79 Cr 0.21 , and 140.2 μΩ•cm for Pt 0.70 Cr 0.30 .…”

“…× 10 5 (ℏ/2e)Ω −1 •m −130 ), large internal spin Hall ratio (θ SH Pt ∼ 0.8 31 ), and low thin-film resistivity (ρ xx Pt = 15−100 μΩ•cm 30 ) that can jointly result in a low power consumption for SOT switching. Several recent reports further pointed out that by introducing 3d transition metals such as Cr into Pt, the overall ξ DL can be enhanced via the orbital Hall effect 32,33 or the intrinsic spin Hall effect. 34 However, adopting Pt-based alloys for BEOL-compatible SOT-MRAM (p-MTJ) devices has been proven challenging, since the performance of the thermally annealed Pt/CoFeB/MgO-based devices is still hampered by two critical issues: (i) hard to realize a decent PMA property 35 of the heterostructure and (ii) the poor spin transparency at the Pt/CoFeB interface 36 All layers are deposited in a magnetron sputter deposition system and experienced the following post-annealing process with a thermal budget of 400 °C for 1 h. Then, these film stacks are patterned into Hall cross devices with a geometry of 60 × 5 μm 2 .…”

“…Besides the abovementioned SCSs, Pt-based alloys , have been considered as the most competitive candidates to replace W due to their large spin Hall conductivity (SHC) (σ SH Pt ∼ 5.9 × 10 5 (ℏ/2e)­Ω –1 ·m –1 ), large internal spin Hall ratio (θ SH Pt ∼ 0.8), and low thin-film resistivity (ρ xx Pt = 15–100 μΩ·cm) that can jointly result in a low power consumption for SOT switching. Several recent reports further pointed out that by introducing 3 d transition metals such as Cr into Pt, the overall ξ DL can be enhanced via the orbital Hall effect , or the intrinsic spin Hall effect . However, adopting Pt-based alloys for BEOL-compatible SOT-MRAM ( p -MTJ) devices has been proven challenging, since the performance of the thermally annealed Pt/CoFeB/MgO-based devices is still hampered by two critical issues: (i) hard to realize a decent PMA property of the heterostructure and (ii) the poor spin transparency at the Pt/CoFeB interface while compared to the more commonly reported Pt/Co (ξ DL Pt/CoFeB = 0.08–0.15 − and ξ DL Pt/Co = 0.1–0.2 − ).…”

Hidden interplay of current-induced spin and orbital torques in bulk Fe$_3$GeTe$_2$

“…Known as the orbital torque, it provides an alternative promising route to control the magnetization through orbital degree of freedom, which has just started to be explored in recent years [32]. So far, the orbital torque has been experimentally observed mostly in transition-metal-based magnetic heterostructures [33][34][35][36][37][38][39][40][41]. However, owing to highly anisotropic crystal field potential, strong spin-orbit coupling (SOC), and orbital complexity of the electronic structure, bulk FGT is expected to exhibit rich entangled dynamics between spin and orbital degrees of freedom.…”