Doctor of Philosophy (Ph.D.)
Muhammad Mumtaz Qazilbash
Seth A. M. Aubin
Charge transport properties of manganites can be significantly modified by temperature, chemical doping, strain, and interfacial boundaries. In this dissertation, we report studies on broadband far-field infrared spectroscopy and near-field infrared imaging of single crystalline thin films of Sr doped manganite LaMnO3 at 0.33 doping level. at this Sr-doping level, the manganite films undergo a phase transition between a ferromagnetic metallic phase at low temperatures to a paramagnetic, insulating phase at higher temperatures. The films were grown on different substrates with different thicknesses by pulsed laser deposition method. The temperature dependent far-field infrared data on 85 nm thick La0.67Sr0.33MnO3 (LSMO) film grown on (100) lanthanum aluminate substrate reveals that electron and hole free carriers behave quite similarly in the low temperature ferromagnetic metallic state of the thin film. The number densities, effective masses and relaxation response of the delocalized electrons and holes are quantified. We discover that only one-third of the doped charges are coherent and contribute to the dc transport. The temperature dependence of the relaxation rate of the free carriers at low temperatures fulfills the formula A+BT2 with anomalously large A and B coefficients compared to a conventional metal like gold. We detected some of the 8 infrared-active phonons predicted for the rhombohedral lattice. We also observed splitting of the 580 cm-1 infrared-active phonon at high temperatures which we attribute to the local Jahn-Teller distortion effect. We performed detailed scattering-type scanning near-field mid-infrared microscopy on an 18 nm thick La0.67Sr0.33MnO3 film grown on (100) strontium titanate substrate. In contrast to a percolative type first-order phase transition, a continuous homogeneous phase transition is observed within the bulk of the thin film when this sample is heated up from room temperature to 330K.The infrared near-field amplitude data is consistent with a second order phase transition from the ferromagnetic metallic phase to the paramagnetic insulating phase. We discover critical fluctuations at a fixed temperature within the bulk of the thin film near its nominal phase transition temperature. We also discover temperature independent phase segregation near the film-substrate interface which we attribute to more conducting regions with A-type antiferromagnetic order coexisting with less conducting ones with C-type antiferromagnetic structure.
© The Author
Xu, Peng, "Infrared Spectroscopy and Nano-Imaging of La0.67Sr0.33Mno3 Films" (2017). Dissertations, Theses, and Masters Projects. Paper 1516639666.