Perpendicular MgO-based magnetic tunnel junctions are optimal candidates as building blocks of spin-transfer torque (STT) magnetoresistive memories. However, up to now, the only STT is not enough to achieve switching current density below \(10^{\mathrm {\mathbf {6}}}\) A/cm \(^{\mathrm {\mathbf {2}}}\). A recent work has experimentally demonstrated the possibility of performing magnetization switching assisted by an electric-field at ultralow current density. Theoretically, this switching has been studied using a macrospin approach only. Here, we show a full micromagnetic study. We found that the switching occurs via a complex nucleation process including the nucleation of magnetic vortexes. © 1965-2012 IEEE.