Bulk spin-orbit torque-driven spin Hall nano-oscillators using PtBi alloys

Abstract

Spin-orbit-torque-driven auto-oscillations in spin Hall nano-oscillators (SHNOs) offer a transformative pathway toward energy-efficient, nanoscale microwave devices for next-generation neuromorphic computing and high-frequency technologies. A key requirement for achieving robust, sustained oscillations is reducing the threshold current ($I_{\text{th}}$), strongly governed by spin Hall efficiency ($\theta_{\text{SH}}$). However, conventional strategies to enhance $\theta_{\text{SH}}$ face trade-offs, including high longitudinal resistivity, interfacial effects, and symmetry-breaking torques that limit performance. Here, we demonstrate a substantial enhancement of the bulk spin Hall effect in PtBi alloys, achieving over a threefold increase in $\theta_{\text{SH}}$, from 0.07 in pure Pt to 0.24 in Pt$_{94.0}$Bi$_{6.0}$ and 0.19 in Pt$_{91.3}$Bi$_{8.7}$, as extracted from DC-bias spin-torque ferromagnetic resonance. The enhanced $\theta_{\text{SH}}$ originates from bulk-dominated, extrinsic side-jump scattering across all PtBi compositions. Correspondingly, we observe a 42\% and 32\% reduction in $I_{\text{th}}$ in 100 nm SHNOs based on Co$_{40}$Fe$_{40}$B$_{20}$(3 nm)/Pt$_{94.0}$Bi$_{6.0}$(4 nm) and Co$_{40}$Fe$_{40}$B$_{20}$(3 nm)/Pt$_{91.3}$Bi$_{8.7}$(4 nm), respectively. Structural characterization reveals reduced Pt crystallinity, along with emergence of preferred crystallographic orientations upon introducing higher Bi concentrations. Together, these results position PtBi alloys as a compelling alternative to conventional 5$d$ transition metals, enabling enhanced $\theta_{\text{SH}}$ and significantly lower $I_{\text{th}}$, thus opening new avenues for energy-efficient neuromorphic computing and magnetic random access memory.