Interfacial and bulk spin Hall contributions to fieldlike spin-orbit torque generated by iridium

Abstract

We present measurements of spin-orbit torques generated by Ir as a function of film thickness in sputtered Ir/CoFeB and Ir/Co samples. We find that Ir provides a dampinglike component of spin-orbit torque with a maximum spin-torque conductivity $\phantom{\rule{0.28em}{0ex}}{\ensuremath{\sigma}}_{\mathrm{DL}}^{\mathrm{eff}}=(1.4\phantom{\rule{0.28em}{0ex}}\ifmmode\pm\else\textpm\fi{}\phantom{\rule{0.28em}{0ex}}0.1)\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.28em}{0ex}}\frac{\ensuremath{\hbar}}{2e}{\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$ and a maximum spin-torque efficiency of ${\ensuremath{\xi}}_{DL}=0.042\ifmmode\pm\else\textpm\fi{}0.005$, which is sufficient to drive switching in a 0.8 nm film of CoFeB with perpendicular magnetic anisotropy. We also observe a surprisingly large fieldlike spin-orbit torque (FLT). Measurements as a function of Ir thickness indicate a substantial contribution to the FLT from an interface mechanism, so that in the ultrathin limit there is a nonzero FLT with a maximum torque conductivity $\phantom{\rule{0.28em}{0ex}}{\ensuremath{\sigma}}_{\mathrm{FL}}^{\mathrm{eff}}=\ensuremath{-}(5.0\phantom{\rule{0.28em}{0ex}}\ifmmode\pm\else\textpm\fi{}\phantom{\rule{0.28em}{0ex}}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{0.28em}{0ex}}\frac{\ensuremath{\hbar}}{2e}{\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$. When the Ir film thickness becomes comparable to or greater than its spin diffusion length, $1.6\ifmmode\pm\else\textpm\fi{}0.3\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, there is also a smaller bulk contribution to the fieldlike torque.

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