The spin Hall effect (SHE), which enables the conversion of charge currents into spin currents in materials with strong spin–orbit coupling (SOC) has been harnessed to induce spin–orbit torque (SOT) and manipulate magnetization in ferromagnetic/nonmagnetic (FM/NM) heterostructures, paving the way for next-generation spintronic memory and logic devices. However, a new frontier has emerged with the exploration of orbital angular momentum in materials in which SOC is not a prerequisite, leading to the development of orbitronic devices. The orbital Hall effect (OHE) generates orbital currents transverse to an applied electric field, which can be converted into spin currents for magnetization switching. Despite recent advances, experimental investigations into magnetization switching through orbital torque (OT) in ferromagnetic layers with perpendicular magnetic anisotropy (PMA) remain limited. Now, writing in Nature Communications, Yuhe Yang et al. address this gap by examining the OT in light metal (LM) Zr systems with PMA FM layers, providing crucial insights into the OT efficiency that could lead to the realization of low energy orbitronic devices.
The researchers focused on the differences between SOT and OT systems, and showed that although SOT efficiency is predominantly influenced by the SOC properties of spin Hall materials, OT efficiency is contingent on both the OHE properties of OHMs and the SOC-induced orbital-to-spin current conversion in FM layers. The findings show that Zr/CoPt heterostructures exhibit superior OT efficiency compared with Zr/CoFeB/Gd/CoFeB structures owing to a combination of high OHE conductivity in Zr and a robust ηL-S value in CoPt. Further investigation into magnetization switching capabilities demonstrates that Zr-based OHMs can facilitate efficient switching at lower current densities than their W-based spin Hall material counterparts. This distinction underscores the pivotal role of ηL-S in determining OT efficiency. The analysis by Yang et al. confirmed that OT mechanisms function within PMA materials when interfaced with Zr OHMs. Moreover, the results suggest that larger ηL-S values within PMA FM layers significantly enhance OT efficiency, thereby offering a more effective route for magnetization switching compared with systems with lower values corroborated by calculated spin–orbit correlation functions that exhibit stronger correlations in CoPt structures relative to CoFeB/Gd/CoFeB structures. Therefore, optimizing PMA FM layers with high ηL-S values could improve device performance while maintaining energy efficiency — a crucial consideration for industrial applications. Further optimization strategies could involve engineering PMA materials with both high ηL-S and low damping constants or exploring synthetic antiferromagnetic structures that combine these desirable traits.
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