Electron paramagnetic studies of diluted magnetic semiconductor nanostructures

In this thesis we investigate the Electron Paramagnetic Resonance (EPR) of Diluted Magnetic Semiconductor (DMS) nanostructures. The first chapter studies the appearance of strain-induced magnetic dipole forbidden transitions in the EPR spectrum of highly mismatched DMS superlattices grown by Molecular Beam Epitaxy (MBE). We investigated ZnTe/MnTe, ZnTe/CdTe:Mn and CdTe/ZnTe:Mn superlattices where the lattice mismatch between layers reaches values as large as 6%. We report the appearance of forbidden transitions even at orientations where strain should not produce any mixing, and interpret them as the onset of three dimensional growth of Quantum Dots (QD). The next chapter investigates EPR as a tool to identify the interaction mechanisms between farther than nearest neighbor magnetic ions in highly diluted DMS epilayers. We show that the extraordinary sensitivity of EPR allows us to use the linewidth information for this purpose, and propose a new method to make this identification based in the three site model of Larson. Finally we performed EPR measurements in a set of $Cd1-xMnxS$ nanoparticles with Mn concentrations x = 17%, 33% and 50% and sizes ranging from about 60 angstroms to 1 micrometer. We have found a very large temperature-dependent splitting of the central resonance for temperatures below 20K associated with the hexagonal phase that is common to the three concentrations. This splitting suggest the presence of large temperature dependent internal fields associated with the uniaxial nature of the crystallographic phase. We propose that the splitting is due to the presence of a pure antiferromagnetic to canted weak ferromagnet phase transition.