Investigation of magnetization dynamics in nanostructures

Abstract

This thesis investigated magnetization dynamics in nanostructures. Magnetization can be considered as an ensemble of large number of spins. In a macrospin model, magnetization is assumed to be spatially uniform so that it can be treated as a classical vector. Typically a Landau-Lifshitz-Gilbert (LLG) equation is used to described the dynamics of magnetization. The LLG equation is usually derived phenomenologically and the parameters of the equation needs to be tuned in order to comply with the experiments. Study of the magnetization dynamics from first principles is still insufficient.

In this thesis, by using the time dependent Green’s function theory, magnetization dynamics is studied in two systems: quantum dot with normal leads, and quantum dot with ferromagnetic leads. Expressions of intrinsic Gilbert damping tensor as well as fluctuating torque are derived in terms of Green’s function. Our expression of Gilbert damping tensor resembles the one derived from scattering matrix theory in the limit of low temperatures and wide band approximation, but is suitable for general case. Spin continuity equation of the system is also discussed, which shows how spin current is included in the equation of magnetization dynamics.

Recently spin torque effect due to the presence of Rashba spin-orbit coupling (RSOC) opens the possibility of a new mechanism to manipulate magnetization. Currently most of the studies focused on infinite two dimensional electron gas (2DEG) system where current is driven by an external electric field. A semiclassical Boltzmann (SCB) transport equation was used and the nonequilibrium spin density was found to be linearly proportional to the charge current density. However, systematic investigation of such effect in a mesoscopic system beyond the semiclassical Boltzmann description has not been reported. It is purpose of this thesis to fill this gap.

In this thesis, magnetization dynamics is investigated in a finite 2DEG system where current is driven by bias instead of electric field. Rashba spin orbit coupling (RSOC) in 2D ferromagnetic materials generates spin polarization in 2DEG, thus a spin torque is induced. Magnetization dynamics of the ferromagnetic 2DEG is investigated in a tight binding model, which shows similar magnetic field assist switching effect as in the experiment. Our formalism is feasible for the first principle calculation of magnetization dynamics.