  We have investigated Surface Acoustic Wave (SAW) induced ferromagnetic
resonance (FMR) assisted Spin Transfer Torque (STT) switching of perpendicular
MTJ (p-MTJ) 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 precesses 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) vary across grains of different anisotropy.
Nevertheless, the high mean deflection angle 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 assisted switching can improve energy efficiency
while being scalable to very small dimensions, which is technologically
important for STT-RAM and elucidates the physical mechanism for the potential
robustness of this paradigm in realistic scenarios with thermal noise and
material inhomogeneity
