Fabrication and characterization of magnetotransport in colossal magnetoresistive manganite thin films and hybrid structures.

摘要

The continually increasing demand for magnetic information storage and retrieval has driven a significant worldwide effort to improve the performance of relevant hard ware components. As the areal density continues to increase, more sensitive materials and innovative structures will be required to detect the decreasing fringe fields emanating from the media. Doped manganites in thin film form are being examined as a possible next generation magnetoresistance sensor material. The magnetoresistance of these doped manganite thin films is of unprecedented magnitude; however, these large resistance changes are achieved only in a strong field in the Tesla range, thus severely limiting their practical utility. This dissertation addresses some of the critical parameters, which influence the properties and efforts to reduce the field scale necessary to observe high magnetoresistance ratios in these films.; The primary deposition technique used in this work is pulsed laser deposition. Initial work was concentrated on the optimization of various process parameters to obtain high quality thin films of manganites. Systematic post deposition heat treatments in oxygen and argon ambient at elevated temperatures revealed that transition temperature is related to the Mn-O fraction and uniform distribution of oxygen across the films rather than just the oxygen content of the films as proposed by others. MR ratio is improved by the improvement in the microstructure (recrystallization and grain growth) of the films after annealing; however vacancies created on the lanthanum site by the high temperature anneal also seem to be an important factor in the determination of the MR ratio.; Self-doped, mixed doped and external doped lanthanum manganite thin films, where the external dopant is Ca, have shown different transition temperature and MR ratios. The variation in the insulator-to-metal transition could be explained on the basis of Mn4+ content while the MR property seemed to be related to the microstructure of the films (impurities, defects, grain boundary and domain wall scattering).; A novel way to reduce the fields necessary to produce high magnetoresistance ratios in doped manganites may be to provide a local biasing field. This local biasing field may be generated either by the addition of a magnetic impurity (such as cobalt in this study) or by another ferromagnetic film in close proximity as that in a superlattice configuration. Indeed, an enhancement in magnetoresistance was observed in both the studies.