Efficient orbital torque in polycrystalline <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mrow><mml:mi>ferromagnetic</mml:mi><mml:mtext>−</mml:mtext><mml:mi>metal</mml:mi></mml:mrow><mml:mo>/</mml:mo><mml:mi>Ru</mml:mi><mml:mo>/</mml:mo><mml:msub><mml:mi>Al</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math> stacks: Theory and experiment
Liyang Liao, Fenghua Xue, Lei Han, Junyeon Kim, Ruiqi Zhang, Lun Li, Jiuming Liu, Xufeng Kou, Cheng Song, Feng Pan, Y. Otani
Abstract
Recently emergent current-induced orbital torque is considered more effective for magnetization switching than the well-established spin-orbit torque. However, long-range orbital transport in polycrystalline films and the theory on orbital transport in polycrystalline heterostructures remain elusive. Here we report a large torque effect in $\mathrm{CoFeB}/\mathrm{Ru}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ polycrystalline stacks. The unfilled $d$-shell and small spin-orbit coupling in Ru provide an ideal platform for orbital generation and transport. The orbital current from the $\mathrm{Ru}/\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$ interface can go through a thick Ru layer, with a peak value at 7-nm-thick Ru, and then induces a strong torque effect in CoFeB. The torque efficiency increases unprecedentedly with increasing CoFeB layer thickness, leveling off at $\ensuremath{\sim}0.3$ for 12-nm-thick CoFeB. Theoretical analysis shows that the orbital transport in polycrystalline materials exhibits a unique random precession behavior, leading to a more efficient orbital transport than that in single crystals. Besides the fundamental significance, our findings advance the development of practical orbital torque devices.