| Transition metal oxides based on perovskite or perovskite-derived structures have gained much attention due to their rich and fascinating properties including electrical, magnetic and optical effects. In particular, as an important member of perovskite-type cobalt oxides, LaCoO3exhibits a variety of unusual properties including complex spin-state transition and spin glass behavior, making it excellent candidate for various devices applied in magnetic sensor, magnetic storage and laser device. Especially, recent studies show that LaCoO3nano-materials and epitaxial thin films exhibit ferromagnetic ordering at low temperatures, which is unobserved in the bulk. However, the origin and exact nature of the spin states presented in LaCoO3epitaxial films are still not fully understood and need further study. Therefore, in this dissertation, LaCoO3polycrystalline and epitaxial nano-thin films were grown on single-crystal Si, SrTiO3and LaAl03substrates by a simple polymer assisted deposition method. Based on the characterizations for the strain-induced structural distortion and ferromagnetism in LaCoO3epitaxial films, the cause of spin-state transition and the origin of ferromagnetism were discussed in order to provide a basis for tuning the electrical and magnetic properties of LaCoO3films. Details are as follows:In chapter one, based on the crystal, electronic and magnetic structures of perovskite-type oxides, some typical physical effects and characteristics existing in perovskite or perovskite-derived metal oxides especially in LaCoO3were introduced. Furthermore, as the main point of focus, the historical and recent researches on LaCoO3bulk, nanoparticles, nanowires as well epitaxial thin films were summarized and reviewed.In chapter two, LaCoO3polycrystalline films deposited on (100) oriented Si substrates were prepared by the polymer assisted deposition method. According to the analyses of crystal structure and magnetic properties, it reveals that the crystal structure of the cobalt oxide film is determined by the structure and surface plane of the substrate, especially for the thiner film. Therefore, when the film is thiner, it tends to form La2CoO4But when the film becomes thick, it tends to form LaCoO3. Because the crystal symmetry and in-plane lattice parameters of La2CoO4are more close to those of Si (100) substrate, compared with those of LaCoO3. It can be found that the200-nm-thick film coated at1500rpm is pure phase (LaCoO3). LaCoO3nanoparticles with a mean particle size～100nm are well-proportioned on the surface of Si (100) substrate. It shows a dense and crack-free surface morphology. At low temperature, it can be considered that LaCoO3polycrystalline film is nonmagnetic with a small amount of paramagnetic phase.In chapter three, LaCoO3epitaxial films with different thickness were grown on (001) SrTiO3substrates by a simple polymer assisted deposition method for the first time. X-ray diffraction analyses indicate that single-phase (001) oriented LaCoO3epitaxial films with a pseudotetragonal structure were grown on (001) SrTiO3substrates successfully, with a biaxial tensile strain and tetragonal distortion. Due to the lattice relaxation effect in epitaxial thin film, the biaxial tensile strain is slightly relaxed when the thickness of the LaCoO3film increases, resulting in an increase of the c-axis constant in contrast to a decrease of the in-plane constants of the film. It is different from LaCoO3bulk with a nonmagnetic ground state that all the epitaxial films exhibit a ferromagnetic transition at Tc～85K. Combining with the structural and magnetic analyses, it is shown that the strain-induced ferromagnetism in LaCoO3epitaxial films, corresponding to the higher spin states, origins from the decrease of the energy difference between eg and t2g levels, which is caused by an increase of the unit-cell volume and suppression of the CoO6octahedral rotations. In addition, the change of FC curve with the thickness of the film reveals that the ferromagnetism is enhanced by the thickness decrease of LaCoO3film due to the increase of the biaxial tensile strain.In chapter four, LaCoO3epitaxial nano-thin films with different thickness were also grown on (100),(110), and (111) oriented LaAl03substrates, respectively, by the polymer assisted deposition method for the first time.The effects of the LaAlO3substrate orientation and the film thickness on the ferromagnetic of the LaCoO3films are discussed in order to understand the nature of the ferromagnetic ordered state. Structural analyses confirm that all the LaCoO3films are epitaxially grown in accordance with the orientation of LaAlO3substrates, with biaxial compressive strain in the ab plane. Due to the different strain directions, the Jahn-Teller-like tetragonal distortion of CoO6octahedron is dominant in LaCoO3film on (100) LaAlO3, but the rotation of CoO6octahedron is dominant in LaCoO3film on (111) LaAlO3, while it is intervenient between them in LaCoO3film on (110) LAO. An obvious ferromagnetic transition at Tc-85K is observed in all the (100) oriented LaCoO3films, but a very weak ferromagnetic signal in (110) oriented LaCoO3films and a nonmagnet-like characteristic in (111) oriented LaCoO3films below～85K. It is found that the change of the ferromagnetic signal is closely related to the stain-induced Jahn-Teller tetragonal distortion of CoO6octahedron which can stabilize Co3+ions in the intermediate spin state. The higher degree of the Jahn-Teller tetragonal distortion of CoO6octahedron corresponds to the higher field-cooled magnetization. However, the rotation mode of CoO6octahedron will weaken the degree of Jahn-Teller tetragonal distortion and reduce the population of Co3+in the intermediate spin state.In chapter five, LaCo1-xFexO3epitaxial nano-thin films were also grown on (100) oriented SrTiO3and LaAlO3substrates, respectively, by the polymer assisted deposition method. The structural and magnetic changes under the dual roles of B-site doping and epitaxial strain for LaCoO3epitaxial films are discussed. It reveals that the unit-cell volume of LaCo1-xFexO3becomes larger when Fe doping content increases. With increasing Fe doping content, the Jahn-Teller tetragonal distortion of BC6octahedron and the epitaxial strain for LaCo1-xFexO3/SrTiO3films are all weakend gradually, but those of LaCo1-xFexO3/LaAlO3films increases gradually. It is found that the magnetization M is strongly lowed by Fe substitution. The antiferromagnetic interactions through Fe-O-Co3+play a main role in LaCo1-xFexO3system, which can suppress the ferromagnetic interactions through Co3+-O-Co3+, obviously. With increasing Fe concentration, the stronger antiferromagnetic exchange interactions through Fe3+-O-Fe3+appear and further suppress the ferromagnetism, resalting in the paramagnetic behavior from10to150K. In addition, the Co3+ion is stabilized in the low spin state and the ferromagnetism is suppressed with Fe doping in B-site. It indicates that the chemical pressure induced by doping has greater effect on the magnetic change in LaCoO3epitaxial film, compared with the epitaxial strain. |
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