Efficient orbital torque in polycrystalline ferromagnetic-metal/Ru/Al2O3 stacks: Theory and experiment

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 CoFeB/Ru/Al2O3 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 Ru/Al2O3 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 ???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.