| Since the1950s, the existence of strong interactions between charge, spin, orbital and lattice in perovskite manganese oxide system lead to a series of fantastic phenomenon, such as metal-insulator transition, charge ordering, phase separation and colossal magnetoresistance et al., which have been attracting great attention. The discoveries of these interesting physical phenomenon are not only greatly enriched the connotation of condensed matter physics, but also contribute to the development of spintronics and strongly correlated electron devices research. But so far, the physical nature of the various phenomena is still not fully understood, due to the complex interactions in the materials systems. Hence, more experimental studies on the perovskite manganese oxide are very necessary.This thesis is focused on the crystal structure and electromagnetic transport properties of La0.625Ca0.375MnO3(LCMO) epitaxial films which are under the strain modulation of substrate. For LCMO films made by pulsed laser deposition (PLD) method, exploration and optimization on preparation process were performed. Phase separation and enhanced magnetoresistance in the strained epitaxial La0.625Ca0.375MnO3(001) films were found. At the same time, we observed annealing induced coherent evolutions of biaxial strain and antiferromagnetic-insulator phase in La0.625Ca0.375MnO3(001) films. On the other hand, we discussed In-plane ordered grain boundaries inducing enhanced magnetoresistance in epitaxial manganite films.This thesis consists of five chapters. The main contents of each chapter are summarized as follows:The first chapter summarizes the research progress and status of the perovskite manganese oxide, including basic properties and theories of perovskite manganese oxide, all kinds of magnetoresistance effects and lattice strain effect on the nature of the manganese oxide films.In chapter2, we have designed and analyzed our experiments by Taguchi method, and achieve the optimized process parameters of PLD method. After all, we obtain three important consequences:(1) we discuss the effects of laser energy, oxygen partial pressure, substrate-target distance and substrate temperature process on the morphology of films;(2) the process parameters for films with high quality and optimized surface roughness have been selected carefully;(3) we found that an effective control of the lattice distortion by the oxygen partial pressure in epitaxial films, and this approach plays an important role in our future work. The study and exploration on the preparation process lay a solid foundation for the physical properties about such materialsIn chapter3, La0.62Ca0.375MnO3(LCMO) films which were deposited on (LaAlO3)0.3(Sr2AlTaO6)0.35(001)[(LSAT)(001)] single crystal substrates with various oxygen partial pressure (20Pa,30Pa and40Pa) during deposition show controlled tetragonal distortion, due to oxygen partial pressure having a regulatory role for lattice distortion of epitaxial films. The structural and transport measurements demonstrate that the films deposited at30Pa and40Pa have a larger lattice strain and distortion, in which enhanced colossal magnetoresistance (CMR) and anisotropic magnetoresistance (AMR) are observed with stable antiferromagnetic-insulator (AFI) phase. We have discussed the possible origin of the enhanced CMR and AMR effect in the strained LCMO films, relating to the strong competition between AFI and ferromagnetic (FM) phase. In addition, we also explained the nonmonotonic behavior of AMR dependence on magnetization.In chapter4, annealing induced coherent evolutions of biaxial strain and antiferromagnetic-insulator phase in Lao.62sCao37sMnO3(001)[LCMO (001)] films were observed, due to the strain-driven orbital ordered state. For all the LCMO films which deposited on LSAT (001) substrates with different thickness, it is confirmed that the annealing induced strain evolution in each film includes strain enhancement and then relaxation with increasing annealing treatments. Moreover, after annealing process, the fluctuation of antiferromagnetic-insulator (AFI) phase shows coherent evolution with strain state in each film due to the strain-driven orbital ordered state.In chapter5, La5/gCa3/8MnO3(LCMO) films with [110]-orientation were epitaxially grown on yttria-stabilized zirconia (111)[YSZ (111)] single crystal substrates. The results of x-ray diffractions confirmed that these LCMO/YSZ films show in-plane ordered grain boundaries, which are attributed to the symmetry mismatch between the films and the substrates. Large magnetoresistance over a broad temperature range was observed in these films due to the grain boundary effects. |
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