Spin currents and magnetization dynamics in multilayer systems

Abstract

In this Thesis the interplay between spin currents and magnetization dynamics is investigated theoretically. With the help of a simple model the relevant physical phenomena are introduced. From this model it can be deduced that in systems with small spin-orbit coupling, current-induced torques on the magnetization require inhomogeneous magnetization textures. For systems exhibiting large spin-orbit coupling, expected to be present in multilayer structures, such torques exist even without gradients in the magnetization direction. We consider current-induced torques in ferromagnetic metals with both Rashba spin-orbit coupling and inhomogeneous magnetization. We first construct all torques that are allowed by the symmetries of the system, to first order in magnetization-direction gradients and electric field. Subsequently, we use a Boltzmann approach to calculate the spin torques that arise to second order in the spin-orbit coupling. We apply our results to current-driven domain-wall motion and find that the domain-wall mobility is strongly affected by torques that result from the interplay between spin-orbit coupling and inhomogeneity of the magnetization texture. In Chapter 4 we consider the spin torques induced by a temperature gradient for systems with strong spin-orbit coupling. Using a fictitious gravitational field the thermal linear response coefficients can be calculated using a Kubo formula. However, without properly accounting for equilibrium components unphysical divergences are encountered. We show how to remove these difficulties and predict how the electrically and thermally induced spin-orbit torques change the resonance amplitudes and frequencies in a ferromagnetic resonance experiment. In Chapter 5 we propose an effect whereby an electric current along the interface between a ferromagnetic and normal metal leads to injection of pure spin current into the normal metal, if the magnetization-direction in the ferromagnet varies along the direction of current. For the specific example of a spiral spin structure, we compute the voltage induced via the inverse spin-Hall effect. Furthermore, we show that this pure spin current leads to modification of the parameters that govern the spin-transfer torques and current-driven domain-wall motion, which can be used to optimize the latter in layered magnetic systems. This effect in principle enables control over the location of spin-current injection in devices. In the last scientific Chapter of the Thesis we study the motion of a magnetic domain wall through a disordered potential. This is done by simulations of the two dimensional random-bond Ising model. The domain walls are driven by an applied field. For low fields the velocity is expected to be described by the creep law. Our results indicate roughness and creep motion exponents that are in agreement with values reported in the literature.