The Symmetry Mismatch And Phase Separation In Epitaxial La_1-xCa_xMnO₃ Thin Films

The coupling among freedoms （as charge, spin, phonon and orbit） in strongly correlated system is always one of the hottest issues in condensed matter physics. It can help us to better understand the inherent interactions in condensed matters, also has a significant meaning on designs of functional materials. With the rapid development of growth technology for single-crystal epitaxial films, many interesting phenomena based on phonon-electron coupling have been discovered recently. The transition temperature of ferroelectricity, ferromagnetism and superconducting can be greatly improved in epitaxial films; the ground state of films with specified chemical composition can be artificially chosen, presenting different behaviors from its bulk counterpart; the coupling in multiferroic can be enhanced in multilayers due to the elastic mechanism. In the meantime, because of the excellent epitaxy of perovskite structure, the all-oxide heterostructures can solve the problems on lattice mismatch and interfaces. So, the research on perovskite materials is of great importance. This thesis focuses on the perovskite CMR （La, Ca）MnO3 films, of which the ground state is controlled via strain states, and the relationship between strain and phase inhomogeneity in strongly correlated system is investigated. Moreover, in the last part of the thesis, the research plan based on LCMO epitaxial system has been roughly discussed.This thesis is divided into 7 chapters.Chapter 1: As the introduction of the whole thesis, we mainly present the research on strain-controlled functional oxide films and recent studies on manganites.First, we simply review the interesting phenomena induced by epitaxial strain in oxide films, proving that the physical properties controlled by strain should commonly exist in strongly coupling system. Further, based on the past studies on manganites, we propose that the strain state, including the magnitude and symmetry of strain filed, can be used to control the physical properties in manganites films.Chapter 2: In this chapter, we summarize all the details on sample preparation and measurement methods, which include pulsed laser deposition （PLD）, x-ray diffraction （XRD）, reciprocal space maps （RSM）, atomic force microscope （AFM）, and low temperature measurement system （SQUID and PPMS） etc.Chapter 3: To La_1-xCa_xMnO₃ （0.3 < x < 0.45） whose bulk ground state is ferromagnetic metal （FMM） phase, the La_1-xCa_xMnO₃/NGO（001） films can exhibit phase separation （PS） of coexistence of FMM and antiferromagnetic insulating （AFI） phase. And the AFI phase can be controlled by annealing temperature, film thickness and Ca doping level, resulting promising low-field magnetoresistance.Chapter 4: Through the contrasting experimental results, we find out that La_0.67Ca_0.33MnO₃（LCMO） films on both LSAT（001） and NGO（110） can preserve their bulk-like FMM ground state under isotropic strain field. While ex-situ annealed films on NGO（100） and NGO（001） with anisotropic strain field, they show phase separation at low temperature. Moreover, via the studies on RSMs the LCMO films on LSAT（001） and NGO（110） with stable FMM ground state can hold the high symmetry structure as in LCMO bulk, while on NGO（001） and NGO（100） the structure of LCMO films can be tuned to low symmetry as in NGO, which proves the crucial role of anisotropic strain field in phase-separated manganite films.Chapter 5: Focusing on a series of LCMO/NGO（001） samples, we systemically study the dymamics of this strain-induced PS. First, the intensive competition between FMM phase and AFI phase in these samples can result in both overshot-like thermal hysteresis and multiple metal-insulating transitions in dyamic PS region, in which the strong relaxation of resistivity has been observed. Second, to the specified sasmple at the same temperature but with the different PS situation, the melting filed is kept same. We suggest that the melting field here should be only determined by strain state in films, which distinguish strain-induced PS system from others.Chapter 6: Through the magnetotransport test under different thermal history, the detail phase diagrams of LCMO films can be plotted. In general, the phase-separated films would experience paramagnetic insulating （PI） phase, FMM phase, AFI-dominated dynamic PS, FMM-dominated dynamic PS and frozen PS on cooling from room temperature. The FMM-dominated and AFI-dominated dynamic PS have never been reported in other PS system, and the strain seems to have little influence on the nature of frozen state.Chapter 7: We show the research plan based on LCMO epitaxial system from the aspects of physical understanding and devices application, which contains the physical anisotropy, microscopy of PS evolution, annealing effect and multilayer devices.