Controlling itinerant spin dynamics for enhanced spin transfer torque

The process of spin transfer torque is based on the conservation of total angular momentum of the carrier spins S and localized spins that constitute the magnetization M. Since the electron transit time through a thin magnetic layer is usually much shorter than the precession period of the magnetization M, electron spin dynamics S=S(t) can be evaluated with a quasi-static exchange field B∥M in the Bloch equations. While the injected spins make rapid rotations around M with frequency γB, they also acquire individually small phase differences due to the dispersion in the electron paths through the magnetic layer. In this work, we reexamine the STT process by explicitly considering the dephasing between the injected electron spins. Our theoretical analysis shows that the ensemble-averaged net torque experienced by the magnet is explicitly dependent on the dephasing time τ^* and transit time t_d, given in an oscillatory form. For the trivial case of a short dephasing time (t^* « t_d), this general expression reproduces the well-known equation for spin torque ~M×(B×M). When τ^* ≥ t_d, the result clearly deviates from the conventional understanding, enhancing or suppressing the torque with a choice of layer thickness. This effect may be particularly prominent in an antiferromagnet.