Effects of spin current in ferromagnets

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] This thesis focuses on the investigation of magnetization dynamics induced by spin transfer torques. The first part of this thesis presents the theoretical and numerical study of domain wall motion in the magnetic nanowire. The second part reports the analysis of spin-current-driven magnetization excitations in ferromagnets. Magnetic domain wall is known as the transition region between magnetic domains whose magnetization pointing in different directions. Via the movement of domain wall, these domains can rearrange themselves in response to the external magnetic field or electric current. Domain wall dynamics is an important issue in study of the dynamic behavior of magnetic systems. Transverse wall and vortex wall are two type of walls mostly observed in magnetic thin films. Analytical models for these two walls are proposed in this thesis, and their dynamic behaviors are studied by these models. Micromagnetic simulations are performed to verify our analytical results. Simulation results also reveal the physical origin of many phenomena observed in experiments. The direct interaction between conducting electrons and local moments polarizes the electron spins, and at the same time, exerts a spin torque on the magnetization. When the current density is increased higher than a critical value, the spin torque is able to drive the magnetic system out of equilibrium. Due to the existence of the magnetic damping, a steady state could still be reached in this fluctuation-dissipation system. Physical properties of this dynamic state are studied and presented.