The Study Of Octahedral Distortion And Structural Phase Transition In Transition Metal Oxide Thin Films

Transition metal oxides（TMO）have played an important role in the studies of the memory and storage devices in these years.Enabled by progressing advances in epitaxial thin film synthesis techniques,the rich and novel physical properties of TMO thin films have been gradually explored.Up to now,strain engineering,interfacial charge transfer and cation doping are general methods to control the emergent physical behaviors in oxide thin films.In this dissertation,we demonstrate two different but highly and promising approaches to modify the crystal structure and physical properties of TMO thin films.First of all,we develop an interfacial symmetry mismatch heterostructure La Ni O₃/Sr Ru O₃,utilizing the polarity discontinuity at interface,and the different octahedral rotation patterns between nonmagnetic La Ni O₃ layer and ferromagnetic Sr Ru O₃ layer.The Ru O₆ octahedral tilt in Sr Ru O₃ layer is strongly suppressed by capping with La Ni O₃ layer,and Sr Ru O₃ layer transits to a tetragonal phase from bulk orthorhombic phase.An enhance electrical conductivity is also found in the bilayer sample.Additionally,we tune the magnetic anisotropy and realize a large perpendicular magnetic anisotropy（PMA）via the octahedral engineering.Secondly,we propose a new routine to manipulate the octahedral distortion in Sr Ru O₃ ultrathin layer using the structural transformation of the adjacent infinite-layered Sr Cu O₂ layers,whose structures are quite sensitive to the film thickness.The oxygen coordination transforms from a Planer-type into a Chain-type upon reducing the Sr Cu O₂ layer thickness.We synthesize two sets of sandwiched Sr Cu O₂/Sr Ru O₃/Sr Cu O₂ trilayers with different crystallographic symmetry of Sr Cu O₂.Different oxygen coordination in Sr Cu O₂ leads to an octahedral distortion in Sr Ru O₃,causing diverse population of low spin states and high spin states Ru⁴⁺ions.We observe an enhanced anomalous Hall resistivity and magnetic anisotropy in the 3 u.c.-thick Sr Cu O₂ trilayer.Finally,we demonstrate a new approach to synthesize high-quality single-phase transition metal oxynitride thin films using a nitrogen plasma-assisted pulsed laser deposition system.We explore the electrical transport properties of transparent conductive oxide La VO₃ and antiferromagnetic insulator Cr₂O₃ after anion engineering.The in-situ nitrogen doping process induces holes into La VO_3-δN_δfilm,resulting a reduction of carrier concentration.Surprisingly,the lattice of La VO_3-δN_δis elongated along out-of plane axis along with a huge-angle nitrogen-oxygen octahedral tilt.These lattice distortions lead to a variation of the band structure and La VO_3-δN_δfilm undergoes the metal-insulator transition as temperature decreasing.Furthermore,we achieve the first-ever ferromagnetic chromium oxynitride film from its parent-form antiferromagnetic chromium oxide.We find that the crystal structure of Cr₂O_3-1.5δN_δfilm transits from trigonal phase to tetragonal phase with a reduced saturation magnetization as increasing the nitrogen doping level.The results are consistent with our theoretical predictions.Finally,we obeserve a large exchange bias effect in[（Cr₂O₃）_N/(Cr₂O_2.85N_0.1)_N]₅ superlattice.Our work illustrates the asymmetric interface engineering and anion engineering are two efficient means to manipulate the physical properties of TMO films and extends our understanding of the relationship between the macroscopic physical properties of thin films and their microscopic structures.We believe that this work will stimulate further explorations on oxide and oxynitride functional devices.