Write error rates of in-plane spin-transfer-torque random access memory calculated from rare-event enhanced micromagnetic simulations

Stochastic magnetization dynamics at non-zero temperatures gives rise to write errors in spin-transfer-torque random access memory (STTRAM). In this paper, the write error rate (WER) of an in-plane STTRAM bit is estimated by extending a previously developed rare-event-enhancement (REE) technique for spin-transfer-torque switching to an in-plane magnet. Reliable calculation of write error rates up to 10-9 is demonstrated with only ∼103 micromagnetic simulations, thereby making an otherwise prohibitively large computational burden tractable. For the in-plane bit studied here, WERs obtained from the REE-enabled micromagnetic simulations are found to be higher than those obtained within a spatially-coherent (macrospin) switching assumption. Spatially-incoherent switching modes of different types are observed to reduce the switching speed. A detailed study of these spatially-incoherent modes reveals that, at lower applied currents, the end mode controls the WER slope, whereas, at higher applied currents, switching via vortices or anti-vortices governs the WER slope. A sharp change in the WER slope is observed when the latter type of excitation begins to dominate the unswitched population. By further improvements to the REE technique to selectively take into account the vortices and the anti-vortices, reliable prediction of WERs for all ranges of current is demonstrated. The results could help explain prior experimental observations. REE techniques also could be useful for magnetic devices other than STTRAM where rare events remain important and impact device performance.