UC Irvine

Manipulation of magnetization dynamics in nanoscale systems is critical for developing energy efficient, fast information processing systems. Spin orbit coupling (SOC), the interaction between an electron's spin and its orbital momentum, produces numerous interesting phenomena that can be used to control these dynamics. This dissertation presents a number of studies utilizing SOC to control spin waves in magnetic nanostructures. First, thin yttrium iron garnet (YIG) films were discovered to carry non-reciprocal spin waves. Characterization of the non-reciprocity reveals that it stems from SOC at the interface between YIG and gadolinium gallium garnet (GGG) substrate. The analysis was repeated in thin GGG/Pt/YIG systems to find that the non-reciprocity increased by 50%. Then, gated devices utilizing the spin flexo-electric interaction (SFEI) on GGG/Pt/YIG were studied. The effect is predicted to yield tuneable non-reciprocity in spin waves. Characterization of propagating spin waves through the electric field gate reveal however that the effect is negligibly small in this system. The phenomenological constant related to SFEI was extracted. In ultra-thin CoFeB nanowire systems, the effects of voltage controlled magnetic anisotropy on propagating spin waves were measured. Specifically, a nanoscale spin wave field effect transistor is realized. In this device, a voltage applied to the gate efficiently modulates the amplitude of spin waves propagating between the source and drain of a ferro-magnetic spin wave channel. Finally, magneto-mechanical nanodevices that allow the study of the effects of magneto-elastic coupling on propagating surface acoustic waves were developed.

In ferromagnet/non-magnetic heavy metal bilayer nanowires, charge current generates a transverse spin current through SOC (called the spin Hall effect). This spin current can apply a torque that negates the damping in a ferromagnet and drives auto oscillations (AOs) which emit microwave power. This dissertation includes two studies regarding these systems. The dimensional crossover of such nanowire systems between quasi-one-dimensional to quasi-two-dimensional wires was investigated. Analysis of the nanowire AOs show that increasing the wire width results in an increase of the number of excited AO modes accompanied by a decrease of the amplitude and coherence of each mode. Thus revealing that there is an optimal wire width that maximizes power output. This is because the increasing number of modes leads to an increase in non-linear interactions between them which lowers the overall amplitude and phase coherence. Such spin Hall oscillator systems however do not have the ideal configuration for maximum torque because their magnetization is not orthogonal to the spin Hall current polarization. A new nanowire system where the magnetization prefers an easy-plane that is orthogonal to the spin torque is realized. This easy-plane configuration exhibits large angle dynamics and enhanced phase coherence. Micromagnetic simulations reveal that this is achieved through balancing the energy landscape of the nanowire such that there is a near-degeneracy along the wire and out of plane.