A simulation study of stand-by and active write mode magnetic immunity of perpendicular spin-transfer-torque random-access memory

Magnetic immunity is an important reliability metric for spin-transfer-torque random-access memory (STT-RAM). The presence of an external magnetic field may cause retention fails in stand-by mode or switching fails during the write operation. Specifically, active write mode magnetic immunity has not been well explored although it was reported to be the limiter in deciding the magnetic immunity metrics. Here, we present a simulation study of stand-by bit error rates (BER) and write error rates (WER) under the influence of external magnetic field perturbation. Results show that the effect of the external magnetic field is more pronounced when it is applied along a direction non-collinear to the easy axis of the magnet. Variation in the stand-by BER is found to follow the Stoner-Wohlfarth model. It is also observed that the active write mode BER may increase by orders of magnitude for specific directions of applied fields depending on the applied write current and magnetic field strength. The variation in WER is explained by the formation of additional zero-torque “stagnation points” on the magnetization unit sphere. The results show the need for careful characterization of both the stand-by mode and the active write mode while measuring the magnetic immunity of the STT-RAM cell.