| In this thesis we explore the epitaxial growth and magnetic properties of GaMnAs epilayers. Much of this work has only recently been made possible by advances in the epitaxial growth of this material using so called the low-temperature molecular beam epitaxy (LT-MBE). We systematically study the effect of the growth condition on the magnetic properties of GaMnAs. We were able to obtain the Curie temperature as high as 110K. The effect of growth conditions such as the growth temperature and As2/Ga ratio is discussed, as well as the origin of ferromagnetic order.;We also use ferromagnetic resonance (FMR) as a tool for characterizing the ferromagnetic GaMnAs thin film. We observed pronounced shifts of the FMR line at the fields above and below the Mn++ (g = 2.00) resonance position for magnetic field orientations perpendicular and parallel to the layer plane. At higher temperatures these shifts decrease rapidly as the Curie temperature is approached, converging on the g = 2.0 resonance position, in full agreement with the effects of demagnetization on FMR in thin film geometry. At low temperatures, however, all specimens show a striking deviation from the simple demagnetization-induced picture, that we ascribe to anisotropy fields and their increasing importance with decreasing temperatures.;In addition, clear spin wave resonances were observed when the applied magnetic field was perpendicular to the film plane. The spin wave spectra show a nearly linear dependence of the resonance mode positions on the mode number, suggesting that the magnetization profiles of the samples are not uniform. A first-order analysis of these effects is presented along with the experimental data.;Additionally, we present our results on attempting to modify the magnetic properties of GaMnAs film by proximity effects, observed when one deposits an antiferromagnetic MnTe overlayer on the ferromagnetic GaMnAs layer. The overlayers were grown directly adjacent to GaMnAs, or were separated from it by non-magnetic GaAs layers. We have observed that the presence of the MnTe overlayer leads to dramatic modifications of the coercive field of GaMnAs. |
