Critical Switching Current in Perpendicular SOT-MRAM Devices

We investigate the magnetization reversal of a ferromagnetic layer by an applied in-plane charge current in SOT-MRAM devices utilizing the spin Hall effect (SHE) as well as the SHE and an applied in-plane external magnetic field. We also consider the case of a strong field-like torque generated by other SOT mechanisms, such as Rashba-type interface effects. By deriving the steady-state solutions of the driven system and evaluating their stability, we identify two distinct mechanisms that govern deterministic switching. In the first-oscillation-driven mechanism, the magnetization is pushed past an unstable steady-state by the initial deviation following the current onset, after which it relaxes towards an alternative steady-state branch in the opposite hemisphere. As lower damping produces a larger initial oscillation amplitude at a given current, the critical current depends on the Gilbert damping constant α. We show that the usual damping-independent critical current expression is recovered in the overdamped regime with α≥1. In the instability-driven mechanism, activated by field-like torque, even a small initial oscillation grows until the equator is crossed and the magnetization ends in the opposite hemisphere. The critical switching current is the minimum of both mechanisms. Thus, the steady-state landscape reveals distinct current regimes, identifying optimal operating regimes for perpendicular reversal.