Impact of net magnetization on spin-orbit torque switching of synthetic ferromagnets in magnetic tunnel junctions

Abstract

Spin-orbit torque (SOT)-induced reversal of a ferromagnet in a magnetic tunnel junction (MTJ) is promising for next-generation novel magnetic memory and spin-logic devices. However, inherent limitations of single ferromagnetic-based free layer (FL) systems are low thermal stability and susceptibility to external magnetic stray fields. To overcome these challenges, synthetic antiferromagnets (SAFs) are integrated into MTJ devices to form hybrid free layers (HFLs); this FL system enables tunability of thermal stability and stray field experienced by the FL, simultaneously lowering the SOT-induced switching currents. Here, we investigate the impact of net magnetization of the SAF-based HFL on the field and current-driven switching behavior. We observe that nearly compensated SAF-HFL systems demonstrate a higher proportion of switching failures at the device level than the uncompensated SAF-HFL systems in both current and field-driven schemes. Micro-magnetic simulations and thin film characterization of SAF-HFL show that the device-level stochasticity arises due to a dynamic decoupling within the nearly compensated SAF systems. We show that optimizing the SAF-HFL systems to have uncompensated magnetization mitigates this stochasticity and improves the SOT switching current for Pt-based SOT channel by at least 20% across the entire tested pulse width range down to 300 ps, thus leading to reliable switching of SOT-MTJs with SAF-HFL stacks.