Quantifying External Magnetic Field Immunity of the Write Process in Perpendicular Spin-Transfer-Torque Random Access Memory

Abstract

The resilience of write operation in spin-transfer-torque random access memory (STT-RAM) under external magnetic field disturbances is often overlooked. In this work, write error rates (WER) of STT-RAM are calculated, including the effects of magnetic field perturbation of varying magnitude and direction. WER is calculated directly from the numerical solution of the two-dimensional Fokker-Planck equation that enables the estimation of ultra-low probability tails of WER. The results show orders of magnitude increase in WER when an external magnetic field is applied along a direction non-collinear with the easy axis of the magnet. Numerical results are explained quantitatively from an analytical theory that confirms the appearance of additional zero-torque “stagnation points” on the unit sphere’s surface, representing the magnetization vector of constant magnitude. Simulated bit error rates are calculated under varying applied current and write pulse duration. Implications on the active write mode magnetic immunity are discussed. The results present critical insights into the switching process of perpendicular STT-RAM memory cells exposed to external magnetic field perturbation and call for a thorough characterization of such failure modes for quantifying external magnetic field tolerance.