Ferromagnetic Resonance Investigation of GaMnAs Nanometric Layers

This thesis is dedicated to the study of the magnetic properties of GaMnAs nanometric layers by the ferromagnetic resonance (FMR) technique. Three series of samples have been studied to investigate independently the influence of the strain, the hole concentration and the Mn concentration on the magnetic properties of GaMnAs. In the first series, the Ga1−xMnxAs samples with x=0.07, grown on GaAs (compressive strain) and GaInAs (tensile strain) substrates are studied. The results of magnetization, resistivity and Hall effect measurements are presented. From the FMR measurements the easy axes of magnetization and the type of magnetic anisotropy are determined. The angular variations of the FMR spectra are studied in detail and the g-factor, Curie temperature and the magnetocrystalline anisotropy constants are determined as function of temperature. Spin wave resonance were equally observed and interpreted. The observations are compared to the proposed phenomenological models and the spin stiffness and the exchange integral between the Mn ions are deduced. The second study concerns a series of GaMnAs samples with the same Mn doping level of 7% atomic concentration in which the hole concentrations was varied via a hydrogen passivation technique. The hole concentrations are deduced from Hall effect measurements in high fields and low temperatures. The measured hole concentrations correspond to different conductivity regimes from insulating to impurity band and metallic regimes. The samples are characterized by SQUID magnetometry and resistivity measurements. The magnetization as a function of hole concentration is compared to the predictions of the RKKY model. ERDA measurements are performed to determine the concentration of hydrogen in the ferromagnetic sample with the lowest hole concentration. The domain structure of this samples is investigated by magneto-optical Kerr effect microscopy. The FMR spectra are analyzed in details and the hole concentration corresponding to the onset of ferromagnetism is estimated to 10cm. The g-factors depend on the hole concentration and temperature. The relation between the g-factors and the theoretically calculated hole polarization of the samples is presented. The anisotropy studies of the samples have provided the investigation of the magnetocrystalline anisotropy constants as a functe l-0 03 29 33 1, v er si on 2 1 D ec 2 00 8 tion of the hole concentration and the temperature. Their variations are compared to the theoretical models. The energy surfaces deduced from the measured magnetocrytalline anisotropy constants are calculated as a function of magnetization and applied field orientations and magnitudes. The influence of increasing the doping level from 7% to 21% atomic concentration is studied in the third series of samples. Contrary to the theoretical predictions, the critical temperature is not increased above 180K. The FMR parameters are compared to those of standard GaMnAs sample doped with 7% atomic concentration of Mn. The reason for no further increase in TC is attributed to high level of magnetic compensation. The measurements are also compared to the theoretical predictions based on the mean field approximations. The relaxation of the magnetization is studied as a function of strain, hole concentration, Mn concentration as well as temperature. The damping constants were found to be anisotropic. This anisotropy however depends strongly on the process whose contribution is dominant for a specific configuration of the system. te l-0 03 29 33 1, v er si on 2 1 D ec 2 00 8