  We propose a method for inducing magnetization reversal using an AC spin
current polarized perpendicular to the equilibrium magnetization of the free
magnetic layer. We show that the critical AC spin current is significantly
smaller than the corresponding DC one. The effect is understood as a
consequence of the underdamped nature of the spin-torque oscillators. It allows
to use the kinetic inertia to overcome the residual energy barrier, rather than
suppressing the latter by a large spin current. The effect is similar to a
swing which may be set into high amplitude motion by a weak near-resonant push.
The optimal AC frequency is identified as the upper bifurcation frequency of
the corresponding driven nonlinear oscillator. Together with fast switching
times it makes the perpendicular AC method to be the most efficient way to
realize spin-torque memory valve.
