Write error ballooning due to anisotropy variation within the free layer in perpendicular magnetic tunnel junctions

Abstract A finite-temperature micromagnetic investigation is conducted to analyze magnetization switching and write error rates in perpendicular spin-transfer torque random access memory (STT-RAM), including the influence of domain wall (DW) pinning arising from variations within the free layer (FL) magnet and stray field from reference layer assembly in the magnetic tunnel junction. The study reveals that uneven perpendicular magnetic anisotropy within the FL could lead to the formation of pinning sites that give rise to metastable states during switching. Such metastable states could result in a significant increase in the write error rates as compared to the ideal situation. Unlike the case with stray fields arising from reference layer assembly, where the DW propagation is hindered only for one switching direction, inhomogeneity within the FL causes the DW pinning to occur in both switching directions. Combined effects of FL inhomogeneity and stray field from the reference layer result in write error ballooning similar to those reported in recent experiments. The impact of free layer inhomogeneity could be further exacerbated by the stray field for one of the switching directions. Surprisingly, the impact of inhomogeneity is also observed to persist at smaller FL dimensions where quasi-coherent switching is expected. These findings provide deeper insights into the potential factors contributing to anomalous write errors in perpendicular STT-RAM.