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The phenomenon of spin transfer torque (STT) has attracted a great deal ofinterests due to its promising prospects in practical spintronic devices. Inthis paper, we report a theoretical investigation of STT in a noncollinearmagnetic tunnel junction under ac modulation based on the nonequilibriumGreen's function formalism, and derive a closed-formulation for predicting thetime-averaged STT. Using this formulation, the ac STT of acarbon-nanotube-based magnetic tunnel junction is analyzed. Under acmodulation, the low-bias linear (quadratic) dependence of the in-plane(out-of-plane) torque on bias still holds, and the $\sin\theta$ dependence onthe noncollinear angle is maintained. By photon-assisted tunneling, thebias-induced components of the in-plane and out-of-plane torques can beenhanced significantly, about 12 and 75 times, respectively. Our analysisreveals the condition for achieving optimized STT enhancement and suggests thatac modulation is a very effective way for electrical manipulation of STT.

Enhancing the spin transfer torque in magnetic tunnel junctions by ac modulation