Magneto-optical spectroscopy of novel ferromagnetic materials
Doctor of Philosophy thesis
Two types of novel ferromagnetic materials, (Ga,Mn)As and Fei/4TaS2, were studied in this dissertation. Interest in (Ga,Mn)As is stimulated by the emerging field of spintronics, which has a potential of bringing a technology revolution in information processing, information storage, and quantum computing. The latter, Feu1/4TaS2, belongs to the family of intercalated transition-metal dichalcogenides (TMDC) with highly anisotropie layered structures. At cryogenic temperatures, ferromagnetic order appears in both materials through the interaction of localized spins and itinerant carriers. In order to investigate these underlying exchange interactions and spin-split band structures, we developed a magneto-optical Kerr effect (MOKE) spectrometer with the full capabilities of magnetic field, temperature, and photon energy scanning. We observed novel and unusual MOKE data as a function of these three continuously tunable parameters. Remanent Kerr angles of (Ga,Mn)As samples showed strong dependence on the photon energy, exhibiting a large positive peak at ∼ 1.7 eV. This peak increased in intensity and blue-shifted with Mn doping and further blue-shifted with annealing. We attribute these changes to the increased hole density and effective Mn content. Our data agree very well with theoretical calculations using a 30-band k · p model with antiferromagnetic p-d exchange interaction without any ad hoc introduction of impurity transitions. The agreement between the data and the model led us to conclude that above-bandgap magneto-optical Kerr rotation in ferromagnetic (Ga,Mn)As is determined by interband transitions. Fe1/4TaS2 exhibited abnormal Kerr hysteresis behavior with a strong sensitivity to the probing photon energy. The abnormal shapes can be fitted with the sum of two error functions, and we provide a tentative physical description based on domain wall physics. However, a few open questions remains, and its microscopic origin is still under investigation. The Kerr spectra were explained by the difference of joint-density-of-state (JDOS) of spin-up and spin-down bands in a simplified model, adopting literature DOS values of Fe1/3TaS2. Accurate simulations require future calculations of the band structure.
Condensed matter physics