| Diluted magnetic semiconductors (DMS) grown by molecular beam epitaxy have been drawing attention in the context of emerging spintronics, which utilizes electron spins to develop devices with new functionalities. The canonical DMS---(Ga,Mn)As---has been on center stage for almost a decade, and extensive efforts have been dedicated to understanding its hole-mediated ferromagnetism, optimizing growth and annealing conditions to achieve higher-Tc, studying the magneto-transport, exploiting its abundant magnetic anisotropy, and so on.;The first chapter provides the motivation for this dissertation and introduces several aspects of the current understanding of (Ga,Mn)As. Both the theoretical models and experimentally established observations are reviewed, focusing on the magnetic and transport properties of (Ga,Mn)As epilayers. Next, the hybrid ferromagnetic metal/semiconductor heterostructures are introduced. As an excellent candidate for making these heterostructures, the semi-metal MnAs is reviewed in terms of its structural and magnetic properties, which are essential for making the exchange-biased devices described in Chapter 5 and Chapter 6.;The second chapter describes the experimental techniques encompassed in the scope of this dissertation. Several important techniques, such as MBE growth, device patterning, magnetometry and transport measurements are discussed.;The third chapter reports the first experiment in this dissertation, which describes the longitudinal magnetoresistance (MR) anomalies of a (Ga,Mn)As epilayer experiencing hard axis magnetization reversal in an perpendicular magnetic field. By probing the MRs for currents running along different crystallographic directions, the origins of these anomalies are identified, and a model based on magneto-impurity scattering is applied to simulate the MR curves at different temperatures.;This dissertation focuses on three aspects of the study of (Ga,Mn)As: (1) Magneto-transport under hard magnetization reversal; (2) Electrical noise properties; and (3) Exchange-biasing and spin-dependent transport in (Ga,Mn)As/MnAs hybrid structures.;The second experiment, described in the fourth chapter, focuses on low frequency electrical noise measurements in (Ga,Mn)As epilayers with different Mn concentrations. The temperature-dependent noise measurements show 1= f-like noise spectra in all the samples, with no observation of any anomalies across the Curie temperatures. However, in the less conducting sample, we find an increase of the integrated noise (over the span 125mHz-11Hz) in the temperature range from 6K to 29K, accompanied by random telegraph noise (RTN) in the time domain. From the magnetic field dependence of the RTN, we infer the existence of nanoscale magnetic clusters, whose fluctuations modulate hole transport.;The fifth and sixth chapters are dedicated to the study of exchange-biasing and spin-dependent transport in (Ga,Mn)As/MnAs hybrid heterostructures. We demonstrate the fabrication of exchange biased devices based upon (Ga,Mn)As by using a hard ferromagnetic metal (MnAs) overlayer that is exchange coupled with a magnetically softer (Ga,Mn)As underlayer. This discovery of exchange biasing offers a new testbed for studying exchange coupling between metals and semiconductors and opens up opportunities for engineering the coercivity of ferromagnetic semiconductors for device applications. In Chapter 5, we first discuss vertical transport in MnAs/(Ga,Mn)As bilayers, demonstrating a "self-exchange biased" spin valve effect in the current-perpendicular-to-the-plane geometry. Next, in Chapter 6, we discuss the transport properties of (Ga,Mn)As-based exchange-biased MTJs, wherein the tunnel magnetoresistance shows evidence for an exchange-spring configuration in the exchange biased (Ga,Mn)As layer. |
