Overcoming the Efficiency-Stability Trade-off in Spin-Orbit Torque Devices with Thermally Robust BCC NiW Alloys

Abstract

The development of high-performance spin-orbit torque (SOT) magnetic memories is fundamentally constrained by a persistent trade-off between spin Hall efficiency, thermal structural stability, and perpendicular magnetic anisotropy in conventional heavy metals. Here, we overcome this limitation by engineering body-centered-cubic (BCC) Ni-doped W alloys as highly efficient and thermally robust spin-current sources. Ni$_{30}$W$_{70}$/CoFeB heterostructures achieve deterministic out-of-plane magnetization switching at an ultra-low critical current density of 1.78 MA/cm$^2$, nearly threefold lower than that of $β$-W, while maintaining a high anisotropy field of 8,500 Oe and a thermal stability factor of 57.9. The BCC Ni$_{30}$W$_{70}$ alloy preserves its structural integrity and the perpendicular magnetic anisotropy of the adjacent CoFeB layer after annealing at 450 $^\circ$C, demonstrating robustness under the stringent thermal processing conditions relevant to back-end-of-line integration. Harmonic Hall and ferromagnetic resonance measurements reveal a large spin Hall angle of -0.39 and a high interfacial spin transparency of 0.75, demonstrating efficient spin-current generation and interfacial transmission. First-principles calculations further reveal enhanced intrinsic spin Hall conductivity in W-rich BCC NiW alloys, associated with the Fermi level lying within a spin-orbit-coupling-induced band gap. These findings establish BCC NiW alloys as a scalable and thermally resilient material platform for energy-efficient SOT-MRAM.