Exchange and crystalline orientation studies of the III-V ferromagnetic semiconductor (Gallium, Manganese) Arsenide

The prototypical dilute ferromagnetic semiconductor (Ga,Mn)As has been extensively studied in the field of spintronics, where both the electron charge and spin are exploited. The origin of ferromagnetism in this material is in the exchange of itinerant holes with localized Mn moments allowing for a direct coupling of both electronic and magnetic properties. Research on (Ga,Mn)As has focused on three areas: 1) the enhancement of Curie temperature, 2) the study of phenomena using this unique material class, 3) development of "proof of concept" devices. This thesis explores each of these three areas.;Significant progress has been made in raising the Curie temperature by increasing both the Mn and carrier concentration, but defect structure plays an ever increasing role as doping is increased. Recent theoretical models and scanning spectroscopy experiments have suggested a novel way of enhancing Tc by exploring the relative microscopic positions of Mn atoms. This thesis discusses work on digital superlattices grown on various crystalline orientations to test this prediction. The unexpected discovery that defect structure is intimately connected to growth direction will also be covered.;Exchange engineering of (Ga,Mn)As is crucial in fabricating certain devices such as low resistance magnetic tunnel junctions (MTJs). This thesis explores exchange biasing with the hard ferromagnetic metal MnAs and shows the coupling can quantitatively be understood in the context of a partial domain wall model. Utilizing this new coupling technique, MTJs were fabricated to explore phenomena such as the metal-insulator transition and temperature dependent tunneling magnetoresistance. Sub-micron sized MTJs were produced for spin transfer torque devices with smaller critical currents than traditional metal based junctions and higher conductances than previously fabricated non-biased semiconductor devices.