  Write asymmetry, the significantly different write current for high-to-low
and low-to-high resistance switching because of natural stochastic behaviors of
magnetization, is a fundamental issue in magnetic random-access memory (MRAM).
For high-performance spin transfer torque (STT) MRAM, it can be eliminated by
precisely controlling atomically thin magnetic multilayers or by introducing
compensation techniques in circuit-level designs, while for spin-orbit torque
(SOT) MRAM, it has not been addressed. Here we systematically investigated the
write asymmetry of SOT-MRAM as a function of applied magnetic fields (H) and
demonstrated that the write currents are intrinsically asymmetric due to
different SOT efficiencies for high-to-low and low-to-high switching.
Furthermore, we found that the SOT efficiency is very sensitive to the tilt
angle between H and write current, which can be tuned through H to achieve
symmetric SOT switching. These results provide an additional guideline for
designing SOT devices and suggest that the write asymmetry can be eliminated by
adjusting the introduced effective magnetic fields within a field-free SOT-MRAM
architecture.
