Generation of high and bidirectional out-of-plane spin–orbit torque through vertical magnetization gradient

Abstract

High efficiency and out-of-plane spin–orbit torque (OOP-SOT) driven magnetization switching is essential for developing spin-based memory and logic devices. In this study, we report the generation of a large charge-to-spin conversion and bidirectional OOP-SOT by engineering a vertical magnetization gradient within a Co/Ho multilayer system. Exploiting the antiferromagnetic coupling between Co and Ho, the magnetization gradient up to 16.8 (emu/cm−3)/nm was achieved by gradually varying the Ho layer thickness from 0.4 to 0.9 nm. The presence of the OOP-SOT was confirmed through Hall resistance-field loop shift measurements, which has been attributed to the broken symmetry in spin current reflection and transmission in the Co/Ho multilayer with vertical magnetic property gradients. Additionally, the effective field of the OOP-SOT is strongly correlated with the direction of the magnetization gradient, measured to be around +0.7 and –2.5 Oe/mA for the positive and negative magnetization gradients, respectively. Furthermore, the largest SOT in the Co/Ho multilayer with a negative gradient was observed compared to the positive gradient and uniform Co/Ho multilayer structures. The enhanced SOT is attributed to the bulk Rashba field generated by the gradient structure, along with increased spin polarization. The results demonstrated here provide a promising approach for utilizing magnetization gradients for the development of efficient, current-driven spin-based storage and logic devices.