paper

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

arXiv:2507.10219

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 ( $I_{th}$ ), strongly governed by spin Hall efficiency ( $I^¸_{SH}$ ). However, conventional strategies to enhance $I^¸_{SH}$ 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 $I^¸_{SH}$ , 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 $I^¸_{SH}$ originates from bulk-dominated, extrinsic side-jump scattering across all PtBi compositions. Correspondingly, we observe a 42\% and 32\% reduction in $I_{th}$ 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 $I^¸_{SH}$ and significantly lower $I_{th}$ , thus opening new avenues for energy-efficient neuromorphic computing and magnetic random access memory.

19 pages, 5 figures