Part I of this thesis describes my experimental and theoretical efforts to understand spin injection into semiconductors. I present extensive discussions on the conditions required to achieve significant spin transfer from a ferromagnetic injector into a paramagnetic conductor.Theoretical calculations for ballistic spin-coupled transport are described. To circumvent conductivity mismatch that occurs for diffusive contacts between ferromagnetic metals and semiconductors, I designed and fabricated new spin injection devices that employ the recently discovered dilute semiconductor (Ga,Mn)As as a spin polarizer. Spin-coupled transport signals were observed in these novel devices.
In the course of this work, I discovered that bulk (Ga,Mn)As itself manifests what we have termed a "giant" planar Hall effect. This magnetoelectronic phenomenon arises from the strong, intrinsic spin-orbit interaction. This phenomenon offers new prospects for applications inmagnetic sensors and storage media, extends the possibility of novel spintronic devices, and enables unprecedented, high resolution measurements of magnetic phenomena. By using the giant planar Hall effect, I have achieved a complete characterization of the magnetic properties of (Ga,Mn)As. This large effect also has enabled the first direct electrical measurements of the propagation of individual domain walls in microdevices. These experiments establish a new approach to the study of ferromagnetodynamics that does not require significant instrumentation. Individual domain walls can be monitored, trapped and manipulated in real time. By suchtechniques, I have been able to directly investigate the resistance ari sing from a single magneticdomain wall for the first time.
Part II describes my studies of the strain-dependent electrical properties of ballistic GaAs two-dimensional electron gases (2DEGs). I have developed techniques to realize freely suspended 2DEGs with moderately high mobility. These have been incorporated into novel GaAs nanoelectromechanical systems that yield high sensitivity for NEMS motion detection.They have also led to my discovery of a new, dipolar mechanism for electromechanical actuation. Suspended quantum dots have also been successfully fabricated using my newly developed freely-suspended 2DEG fabrication methods. These demonstrate pronounced charging effects, even at elevated temperature, and offer new prospects for observing electronic interactions with confined phonons.
