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Spin angular momentum transfer, or spin-transfer, describes the transfer of spin angular momentum between a spin-polarized current and a ferromagnetic conductor. The angular momentum transfer exerts a torque (spin-current induced torque, or spin-torque) on the ferromagnetic conductor. When its dimensions are reduced to less than 100 nm, the spin-torque can become comparable to the magnetic damping torque at a spin-polarized current of high current density (above 106 A/cm2), giving rise to a new set of current-induced dynamic excitation and magnetic switching phenomena. This has now been definitively observed in sub-100-nm current-perpendicular spin-valves and magnetic tunnel junctions, and appears promising as a basis for direct write-address of a nanomagnetic bit when the lateral bit size is reduced to well below 100 nm. An overview is presented in this paper of spin-transfer phenomena. The first part of the paper contains a brief introduction to spin-transfer, especially the characteristic dynamics associated with spin-torque. In the second part, several representative experiments are described. In the third part, a set of basic phenomenological models are introduced that describe experimental observations. The models also serve as a bridge for quantitative comparison between experiments and first-principles spin-polarized transport theory. In the last part of the paper, some device concepts based on spin-transfer-induced magnetic excitation and magnetic reversal are described.
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