Low-magnetic field, room-temperature colossal magnetoresistance in manganite thin films

The manganese (Mn) based perovskite oxide materials (manganites), of the chemical form T_1-−xD_xMnO₃, display a large magnetic field induced decrease in their resistivity, termed colossal magnetoresistance. Typically, colossal magnetoresistance in the manganite samples is observed at low temperatures and high magnetic fields (>1 Tesla). However, an enhanced magnetoresistance at low magnetic fields and room temperature in these manganite samples would be technologically useful. In an effort to characterize this low magnetic field, room temperature magnetoresistance, the role of several different physical parameters has been explored in this thesis. These physical parameters include lattice mismatch strain, which originates from the epitaxial growth of single layer manganite thin films, the application of different radiation probes, such as microwave radiation, and the introduction of artificial grain boundaries in the form of interfaces in manganite multilayers.; Lattice mismatch strain originates from the difference in the lattice constants of the manganite thin film and the crystalline substrate. The nature of the effect of the lattice mismatch strain on these transport properties for La_0.7Ba_0.3MnO₃ thin films has been studied by varying the degree of lattice mismatch strain in the thin film. Variation of the lattice mismatch strain was achieved by varying the thickness of the manganite thin films, by annealing the manganite thin films in oxygen, and by buffering the manganite films with a lattice matched buffer layer. Each of these approaches relaxed the lattice mismatch strain, resulting in an increase of the low magnetic field, room temperature magnetoresistance.; Microwave radiation probes determine the magnetic homogeneity of the manganite thin films and the effect of this magnetic homogeneity on the low magnetic field, room temperature magnetoresistance. La_0.7Ba _0.3MnO₃ thin films showed no gross magnetic homogeneitiese. The magnetic homogeneity increased in the La_0.7Ba_0.3MnO ₃ thin films when the lattice mismatch strain in the film decreased. In addition, as the crystalline quality and the magnetic homogeneity of the thin films increased, the microwave magnetoresistance also increased.; The decrease of the lattice mismatch strain in the manganite thin films ultimately led to an increase in the low magnetic field, room temperature magnetoresistance. However, this enhancement was minimal, so artificial grain boundaries in the form of interfaces in manganite spin valves and spin dependent tunneling junctions were introduced. However, no room temperature, low magnetic field magnetoresistance was observed in either device, due to the decrease of the manganite spin polarization with increasing temperature, the occurrence of spin flip scattering events at defect sites in the multilayer structure, and reorganization of the magnetization at the interfaces. (Abstract shortened by UMI.)...