Achieving low write error rates in perpendicular spin–orbit torque MRAM: A micromagnetic modeling feasibility study

We report a micromagnetic modeling study of switching perpendicularly magnetized free layers with spin–orbit torques (SOTs) generated by spin currents consisting of only orthogonal (conventional SOT) as well as both orthogonal and collinear (unconventional SOT) spin polarizations relative to the free layer (FL) magnetization. Focusing on the FL diameter of 50 nm, for conventional SOT, we find relatively high probabilities of FL back-hopping or incomplete switching at room temperature (T) for several bias configurations that deterministically switch the FL at T = 0 K, including in-plane bias field with and without Dzyaloshinskii–Moriya interaction (DMI) as well as the field-free case with DMI in the presence of field-like torque. For unconventional SOT, fast, field-free deterministic switching is found at T = 0 K for orthogonal y-spin SOT efficiency θy = 0.3 when collinear z-spin efficiency θz/θy > 0.25. Such large ratio is needed to change the nature of the magnetization switching from a non-uniform state with dominant component along y to an almost fully reversed and uniform state along -z, resulting in thermally robust and complete switching without back-hopping. In addition, the presence of z spins of such magnitude results in lowering of the switching current density by a factor of about 3, depending on the damping parameter α of the FL (more for lower α). Based on these results, we conclude that achieving low write error rate operation in perpendicular SOT MRAM is not feasible using approaches that exploit conventional SOT, but it should be straightforward to achieve with unconventional SOTs that meet parameter values determined by this study.