Enhanced anti-damping torque in double-Spin-Hall trilayers

In magnetic thin-film heterostructures, current-induced anti-damping torque can switch magnetization [1], drive domain walls [2], and induce precessional dynamics [3]. The spin Hall effect in ferromagnet/normal-metal bilayers is an especially promising mechanism for generating a robust anti-damping torque. We report on enhanced tuning of resonant magnetization dynamics in in-plane magnetized Ta/CoFeB/Pt trilayers, where both the Ta and Pt layers serve as spin-Hall sources. The change in resonant linewidth induced by in-plane DC current is measured using spin-torque ferromagnetic resonance (FMR) [4] and cavity-based FMR [5]. With optimized Ta and Pt layer thicknesses, we observe in 4-nm thick CoFeB a damping modification of $> 3\times10^{-3}$ per $10^{11}$ A/m$^2$ of DC current, effectively more than doubling the anti-damping torque compared to conventional spin-Hall bilayers. This finding presents a new possibility for increasing the efficiency of spin-Hall driven devices.\\[4pt] [1] L. Liu et al. Science. 336, 555 (2012).\\[0pt0 [2] S. Emori et al. Nat. Mater. 12, 611 (2013).\\[0pt] [3] V. E. Demidov et al. Nat. Mater. 11, 1028 (2012).\\[0pt] [4] L. Liu et al. Phys. Rev. Lett. 106, 036601 (2011).