Strain Control And The Induced Phase Separation And Anistropic Properties Of Epitaxial La_1-xCa_xMnO₃Thin Films

For the past decades, the researches in perovskite heterostructures have flourished. The interest in these epitaxial systems arises from the fact that they have abundant physics and potential applications in electronic devices. Moreover, with the rapid development of heteroepitaxial technology and in-situ characterization of the growth dynamics, the perfect heterointerfaces have been realized in perovskite heterostructures. By stacking different perovskites into epitaxial films, multilayers, and superlattices, the reconstructions of charge, orbital, spin, and lattice degrees of freedom on the nanometer scale may allow not only their properties to be combined but, sometimes, also totally new phenomena to be induced at the heterointerface.The perovskite manganite is one of the most important materials for fabricating the perovskite heterostructures. On one hand, this material has complex phase diagram and interesting phenomena including the colossal magnetoresistance, charge/orbital ordering, and various magnetic structures. On the other, because of the strong coupling between the spin, charge, orbital, and lattice degrees of freedom, the physical properties in manganites can be easily tuned by external stimuli such as the epitaxial strain. Therefore, investigating the structure-properties in manganite-based heterostructures is very important for the further understanding the emergent phenomena at perovskite heterointerfaces.This thesis focuses on the perovskite La0.67Ca0.33MnO3epitaxial films grown on the NdGaO3substrates with various orientations. In these epitaxial systems, the different orthorhombic lattice distortion between the film and substrate can trigger various strain states. Especially for the (001)-and (100)-oriented film, the anisotropic strain can give rise to an evident phase separation (PS) with the coexistence of ferromagnetic-metal and antiferromagnetic-insulating phase. In this thesis, we first characterized the phase instability and probed the origin of the PS. Then we focus on the anisotropic properties in these anisotropically-strained films. And we suggest that the anisotropic strain induced orientation-preferred PS pattern should play a crucial role in triggering these anisotropic properties. In the last part of the thesis, we try to further observe the microscopic PS pattern via the electronic-spin-resonance spectra and magnetic force microscopy techniques.