American Journal of Nano Research and Applications 2018; 6(1): 21-33 http://www.sciencepublishinggroup.com/j/nano doi: 10.11648/j.nano.20180601.13 ISSN: 2575-3754 (Print); ISSN: 2575-3738 (Online) Spin Transport and Dynamics in Multilayer Magnetic Nanostructures Andrii Korostil * , Mykola Krupa Department of Magnetic Mesoscopic Materials and Nanocrystalline Structures, Institute of Magnetism, Kyiv, Ukraine Email address: * Corresponding author To cite this article: Andrii Korostil, Mykola Krupa. Spin Transport and Dynamics in Multilayer Magnetic Nanostructures. American Journal of Nano Research and Applications. Vol. 6, No. 1, 2018, pp. 21-66. doi: 10.11648/j.nano.20180601.13 Received: March 26, 2018; Accepted: April 13, 2018; Published: May 11, 2018 Abstract: The interconnection between the spin current and spin dynamics via the spin-dependent scattering and an accompanying by spin torque effect in ferromagnetic/normal metal based magnetic multilayer nanostructures is studied including a high fast out-of-equilibrium spin dynamics. Features of the spin transport through interfaces and its impact on spin dynamics are described on the base of the scattering matrix formalism for spin flows. The dependence of the spin torque effect on conductance character of the normal metal layers is considered. The exchange processes between the itinerant s and the localized d electrons are described by kinetic rate equations for electron-magnon spin-flop scattering. It is shown that the magnon distribution function remains nonthermalized on the relevant time scales of the demagnetization process, and the relaxation of the out-of-equilibrium spin accumulation among itinerant electrons provides the principal channel for dissipation of spin angular momentum from the combined electronic system. Keywords: Magnetic Nanostructures, Spin Transport, Scattering, Spin Torque Effect, Electron-Magnon Spin-flop Scattering, Nonequilibrium Spin Dynamics 1. Introduction Stacks of alternating ferromagnetic and nonmagnetic metal layers exhibit giant magnetoresistance (GMR), because their electrical resistance depends strongly on whether the moments of adjacent magnetic layers are parallel or antiparallel. This effect has allowed the development of new kinds of field-sensing and magnetic memory devices [1]. The cause of the GMR effect is that conduction electrons are scattered more strongly by a magnetic layer when their spins lie antiparallel to the layer’s magnetic moment than when their spins are parallel to the moment. Devices with moments in adjacent magnetic layers aligned antiparallel thus have a larger overall resistance than when the moments are aligned parallel, giving rise to GMR. At the same time, there is the converse effect: just as the orientations of magnetic moments can affect the flow of electrons, a polarized electron current scattering from a magnetic layer can have a reciprocal effect on the moment of the layer. As shown in [2, 3], an electric current passing perpendicularly through a magnetic multilayer may exert a torque on the moments of the magnetic layers. This effect which is known as ‘‘spin transfer,’’ may, at sufficiently high current densities, alter the magnetization state. It is a separate mechanism from the effects of current induced magnetic fields. Experimentally, spin-current-induced magnetic excitations such as spin- waves, [4-9] and stable magnetic reversal [6, 7] have been observed in multilayers, for current densities greater than 10 7 A/cm 2 . The spin-transfer effect offers the promise of new kinds of magnetic devices and serves as a new means to excite and to probe the dynamics of magnetic moments at the nanometer scale. In order to controllably utilize these effects, however, it is necessary to achieve a better quantitative understanding of current-induced torques. A derivation of spin-transfer torques using a one-dimensional (1D) WKB approximation with spin-dependent potentials presented in [3] only take into account electrons which are either completely transmitted or