We propose a method for inducing magnetization reversal using an AC spincurrent polarized perpendicular to the equilibrium magnetization of the freemagnetic layer. We show that the critical AC spin current is significantlysmaller than the corresponding DC one. The effect is understood as aconsequence of the underdamped nature of the spin-torque oscillators. It allowsto use the kinetic inertia to overcome the residual energy barrier, rather thansuppressing the latter by a large spin current. The effect is similar to aswing 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 ofthe corresponding driven nonlinear oscillator. Together with fast switchingtimes it makes the perpendicular AC method to be the most efficient way torealize spin-torque memory valve.
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