Spin transport and current induced magnetization dynamics in magnetic nanostructures.

Abstract

The study of the interaction between conducting electrons and magnetization in a ferromagnet has stimulated much interest following the discovery of the giant magnetoresistive effect two decades ago. With the advance of fabrication techniques at the nanometer length scale, a variety of new magnetic nanostructures have emerged. These structures are interesting from both a scientific and technological perspective. Some of them have successfully led to applications in information storage industry. This thesis theoretically studies some of these structures and focuses on two aspects: (1) the effect of surface roughness in magnetoresistive devices, (2) spin transfer torque induced magnetization dynamics. Surface roughness is known to be an important source of scattering in small structures. We employ Landauer's formalism to study spin dependent electron transport in structures like spin valve, magnetic tunnel junction and nanowires. An efficient algorithm is developed to solve the scattering problem numerically. It is found that the resistivity and magnetoresistance are strongly influenced by the surface roughness scattering. The coupling between spin polarized current and local magnetic moment results in a torque on the magnetization. This induces dynamic effects such as magnetization reversal and switching. We propose an exchange coupled composite structure to study current induced reversal and show that this structure can significantly reduce the critical current. The spin torque can cancel the damping torque and induce steady precession. This type of spin torque oscillator is attractive as a microwave device at the nanoscale. Several of these oscillators can couple together and oscillate in a phase coherent manner. The mechanism for the coupling is studied analytically and using micromagnetic simulation. It is found that the coupling exhibits an oscillatory behavior through a spin wave mediated interaction.