Acoustic-Wave-Induced Ferromagnetic-Resonance-Assisted Spin-Torque Switching of Perpendicular Magnetic Tunnel Junctions with Anisotropy Variation

We investigate surface-acoustic-wave- (SAW) induced ferromagnetic-resonance- (FMR) assisted spin-transfer-torque (STT) switching of perpendicular magnetic tunnel junctions (PMTJs) with inhomogeneities using micromagnetic simulations that include the effect of thermal noise. With suitable frequency excitation, the SAW can induce ferromagnetic resonance in magnetostrictive materials, and the magnetization can precess in a cone with high deflection from the perpendicular direction. With incorporation of inhomogeneity via lateral anisotropy variation as well as room-temperature thermal noise, the magnetization precession in different gains can be significantly incoherent. Interestingly, the precession in different grains are found to be in phase, even though the precession amplitude (angle of deflection from the perpendicular direction) varies across grains of different anisotropy. Nevertheless, the high mean deflection angle due to acoustically induced FMR can complement the STT switching by reducing the STT current significantly; even though the applied stress induced change in anisotropy is much lower than the total anisotropy barrier. This work indicates that SAW-induced FMR-assisted switching can improve energy efficiency while being scalable to very small dimensions, which is technologically important for STT random-access memory and elucidates the physical mechanism for the potential robustness of this paradigm in realistic scenarios with thermal noise and material inhomogeneity.