  Spin-orbit torque (SOT) represents an energy efficient method to control
magnetization in magnetic memory devices. However, deterministically switching
perpendicular memory bits usually requires the application of an additional
bias field for breaking lateral symmetry. Here we present a new approach of
field-free deterministic perpendicular switching using a strain-mediated SOT
switching method. The strain-induced magnetoelastic anisotropy breaks the
lateral symmetry, and the resulting symmetry-breaking is controllable. A finite
element model and a macrospin model are used to numerically simulate the
strain-mediated SOT switching mechanism. The results show that a relatively
small voltage (${\pm}0.5$ V) along with a modest current ($3.5 \times 10^{7}
A/cm^{2}$) can produce a 180{\deg} perpendicular magnetization reversal. The
switching direction (up or down) is dictated by the voltage polarity (positive
or negative) applied to the piezoelectric layer in the magnetoelastic/heavy
metal/piezoelectric heterostructure. The switching speed can be as fast as 10
GHz. More importantly, this control mechanism can be potentially implemented in
a magnetic random-access memory system with small footprint, high endurance and
high tunnel magnetoresistance (TMR) readout ratio.
