Write error rate of spin-transfer-torque random access memory including micromagnetic effects using rare event enhancement

TL;DR

This paper introduces a rare event enhancement method to efficiently calculate extremely low write error rates in spin-transfer-torque RAM, accounting for micromagnetic effects, with significantly fewer simulations than traditional approaches.

Contribution

The authors develop and demonstrate a rare event enhancement technique tailored for STT-RAM, enabling accurate WER estimation at 10$^{-9}$ with far fewer simulations.

Findings

Reliable WER calculation to 10$^{-9}$ achieved with ~10$^{3}$ simulations.

Micromagnetic effects influence switching dynamics and error rates.

The method can potentially be extended to even lower error rates.

Abstract

Spin-transfer-torque random access memory (STT-RAM) is a promising candidate for the next-generation of random-access-memory due to improved scalability, read-write speeds and endurance. However, the write pulse duration must be long enough to ensure a low write error rate (WER), the probability that a bit will remain unswitched after the write pulse is turned off, in the presence of stochastic thermal effects. WERs on the scale of 10$^{-9}$ or lower are desired. Within a macrospin approximation, WERs can be calculated analytically using the Fokker-Planck method to this point and beyond. However, dynamic micromagnetic effects within the bit can affect and lead to faster switching. Such micromagnetic effects can be addressed via numerical solution of the stochastic Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation. However, determining WERs approaching 10$^{-9}$ would require well over 10$^{9}$ such independent simulations, which is infeasible. In this work, we explore calculation of WER using "rare event enhancement" (REE), an approach that has been used for Monte Carlo simulation of other systems where rare events nevertheless remain important. Using a prototype REE approach tailored to the STT-RAM switching physics, we demonstrate reliable calculation of a WER to 10$^{-9}$ with sets of only approximately 10$^{3}$ ongoing stochastic LLGS simulations, and the apparent ability to go further.