Thermal stability and magnetization switching in perpendicular magnetic tunnel junctions

A micromagnetic study of the thermal stability and magnetization switching by spin-polarized current in Perpendicular Magnetic Tunnel Junctions comprising a four-layer stack of free layer, reference layer, and synthetic antiferromagnets is presented. It is demonstrated that the minimal energy path (MEP)—related to the thermal stability of the system—does not necessarily follow the same trajectory as the current-induced switching. The MEP can be by uniform rotation (UR) or by domain walls (DWs), whereas the current-induced switching can be by UR, DWs, or bubble formation depending on geometrical and stack parameters. The bubble formation is further affected by the direction of switching, i.e., parallel to antiparallel vs antiparallel to parallel switching. We demonstrate the existence of different regimes where the energy barrier and the figure of merit, defined as a ratio between the energy barrier and switching current, show characteristic dependencies and even optimum points depending on the device diameter and the stack parameters. The presented results may explain recent experiments.