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 $\ensuremath{\beta}$ phase, $\ensuremath{\beta}$-$\mathrm{W}$, has the largest reported antidamping SOT charge-to-spin conversion ratio $({\ensuremath{\theta}}_{\mathrm{AD}}\ensuremath{\approx}\ensuremath{-}60\mathrm{%})$ for heavy metals. However, $\ensuremath{\beta}$-$\mathrm{W}$ has a limitation when one is aiming for reliable technology integration: the $\ensuremath{\beta}$ phase is limited to a thickness of a few nanometers and enters the $\ensuremath{\alpha}$ phase above 4 nm in our samples when industry-relevant deposition tools are used. Here, we report our approach to extending the range of $\ensuremath{\beta}$-$\mathrm{W}$, while simultaneously improving the SOT efficiency by introducing $\mathrm{N}$ and $\mathrm{O}$ doping of $\mathrm{W}$. Resistivity and XRD measurements confirm the extension of the $\ensuremath{\beta}$ phase from 4 nm to more than 10 nm, and transport characterization shows an effective SOT efficiency larger than $\ensuremath{-}44.4\mathrm{%}$ (reaching approximately $\ensuremath{-}60\mathrm{%}$ for the bulk contribution). In addition, we demonstrate the possibility of controlling and enhancing the perpendicular magnetic anisotropy of a storage layer ($\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$). Further, we integrate the optimized $\mathrm{W}(\mathrm{O},\mathrm{N})$ 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 $\mathrm{W}(\mathrm{O},\mathrm{N})$ compared with our standard $\mathrm{W}$. Our results open the path to using and further optimizing $\mathrm{W}$ for integration of SOT-MRAM technology.