Bulk Spin-Orbit Torque-Driven Spin Hall Nano-Oscillators Using PtBi Alloys with Engineered Crystallinity

Spin-orbit-torque-driven auto-oscillations in spin Hall nano-oscillators (SHNOs) provide a promising route toward energy-efficient, nanoscale microwave devices for neuromorphic computing and high-frequency technologies. Achieving robust oscillations requires lowering the threshold current (Ith), governed by the spin Hall efficiency (θSH). Conventional approaches to enhance θSH often involve trade-offs, such as increased resistivity and interfacial effects. Here, we demonstrate a pronounced enhancement of the bulk spin Hall effect in PtBi alloys via crystallographic engineering, achieving a 3-fold increase in θSH from 0.07 in Pt100.0Bi0.0 to 0.24 in Pt94.0Bi6.0 and 0.19 in Pt91.3Bi8.7, extracted from DC-bias spin-torque ferromagnetic resonance. The enhancement arises from bulk-dominated extrinsic side-jump scattering. Correspondingly, Ith is reduced by 42% and 32% in 100 nm SHNOs based on Co40Fe40B20(3 nm)/Pt94.0Bi6.0(4 nm) and Co40Fe40B20(3 nm)/Pt91.3Bi8.7(4 nm), respectively. The devices exhibit narrower linewidths (∼25 MHz), enhanced quality factors (350 ≤ Q ≤ 550, ∼4× higher than Pt (Pt100.0Bi0.0)), and a 61.6% reduction in threshold power. These findings establish PtBi alloys as efficient spin Hall materials, enabling reduced power consumption for SHNO-based neuromorphic and memory technologies.