Optimization of Tungsten β -Phase Window for Spin-Orbit-Torque Magnetic Random-Access Memory

Abstract

Switching induced by spin-orbit torque (SOT) is being vigorously explored, as it allows the control of magnetization using an in-plane current, which enables a three-terminal magnetic-tunnel-junction geometry with isolated read and write paths. This significantly improves the device endurance and the read stability, and allows reliable subnanosecond switching. Tungsten in the β phase, β-W, has the largest reported antidamping SOT charge-to-spin conversion ratio (θ AD ≈ −60%) for heavy metals. However, β-W has a limitation when one is aiming for reliable technology integration: the β phase is limited to a thickness of a few nanometers and enters the α phase above 4 nm in our samples when industryrelevant deposition tools are used. Here, we report our approach to extending the range of β-W, while simultaneously improving the SOT efficiency by introducing N and O doping of W. Resistivity and XRD measurements confirm the extension of the β phase from 4 nm to more than 10 nm, and transport characterization shows an effective SOT efficiency larger than −44.4% (reaching approximately −60% for the bulk contribution). In addition, we demonstrate the possibility of controlling and enhancing the perpendicular magnetic anisotropy of a storage layer (Co-Fe-B). Further, we integrate the optimized W(O, N) into SOT magnetic random-access memory (SOT-MRAM) devices and project that, for the same thickness of SOT material, the switching current decreases by 25% in optimized W(O, N) compared with our standard W. Our results open the path to using and further optimizing W for integration of SOT-MRAM technology.

Tailoring the switching efficiency of magnetic tunnel junctions by the fieldlike spin-orbit torque

“…When the two fields add together, the switching is favored, in agreement with previous results obtained in Ta-based samples [25,26]. Ta is known to induce strong FLT, with β ranging from 0.7 to over 4 [17-19, 26, 35, 37, 48], whereas β is typically less than 0.4 in the case of W [33,34]. Thus, it may seem surprising that V c changes by a similar amount with ϕ in Figs.…”

“…The resistivity of the W, Ta, and CoFeB layers is estimated to be 160 µΩ cm, 210 µΩ cm, and 120 µΩ cm, respectively. The efficiency of the DLT (FLT) obtained from harmonic Hall measurements [17] are ξ DL(FL) = −0.33 ± 0.03(−0.10 ± 0.02) for the W-based [34] and ξ DL(FL) = −0.11 ± 0.01(−0.11 ± 0.03) for the Ta-based samples. In both types of samples, the free layer has an easy axis along z and its magnetization can be reversed between up and down states without any stable intermediate levels [Fig.…”

“…The increased susceptibility to switching errors, observed in strong-FLT samples, was attributed to the domain-wall reflection from the edges of the magnetic structure [26,30]. Thus, whereas magnetization reversal is relatively well understood in SOT materials in which the DLT dominates over the FLT, such as Pt [9,17,31] and W [32][33][34], strong-FLT materials, such as Ta [17][18][19]35], Hf [32,[35][36][37] and topological insulators [38][39][40], offer additional opportunities to tune the switching efficiency. Exper-imental work addressing the FLT, however, has only focused on the switching of relatively large (µm-scale) structures, and used either Pt [23,24] or Ta [25,26,30] as the SOT source.…”

Ferrimagnetic Dynamics Induced by Spin‐Orbit Torques

“…Despite this intrinsic complexity, there exist general physical relations between the duration t p and the minimum amplitude J of the electric current (density) that induces the switching. [ 55,60,61 ] In the short‐pulse regime ( t p ≲ 10 ns), Joule heating plays a limited role and the switching is governed by the conservation of the interfacial angular momentum transferred to the magnetic system, such that: [ 45,50,55,56,62 ] $$\[J = {J_0} + \frac{q}{{{t_{\rm{p}}}}}\]$$Here, J 0 is the threshold switching current density in the limit t p → ∞, and q is an effective electric charge density proportional to the required angular momentum. Equation () has important implications.…”

“…Despite this intrinsic complexity, there exist general physical relations between the duration t p and the minimum amplitude J of the electric current (density) that induces the switching. [55,60,61] In the short-pulse regime (t p ≲ 10 ns), Joule heating plays a limited role and the switching is governed by the conservation of the interfacial angular momentum transferred to the magnetic system, such that: [45,50,55,56,62]…”