Orbit Related Magnetic And Electric Properties Of Perovskite Oxide Films

Perovskite oxide films,with the advantage of excellent orbit related magnetic and electric properties,are highly expected for the application in the next-generation high-speed,high-density,and low-power nonvolatile memory.The 3d orbit of perovskite-oxide gives rise to the rich properties of this oxide.How to effectively manipulate their orbit related magnetic and electric property is the key to the practical application of oxide-based electronic devices.This dissertation proposes the main idea of manipulating orbit related magnetic and electric property of manganite and nicklate oxides by interface engineering,strain engineering and electric field.The modulation of crystal structure,electronic phase,orbital occupancy and the related magnetic and electric performance were systemically studied,based on which the mechanism of regulating the orbit related magnetic and electric property was proposed.LaMnO₃ film surface with dominant 3z²-r² orbital occupancy would lead to the degraded ferromagnetism of LaMnO₃ films with decreasing thickness below a critical thickness,below which the magnetization is nearly unobservable.To avoid this phenomenon,we capped LaMnO₃ films with SrTiO₃ films to obtain x²-y² orbital occupancy at the interface,which could enhance the in-plane magnetic coupling.With this method,the magnetization of thin LaMnO₃ films with thickness less than 6 unit cells was tripled,and the critical thickness of manganites was successfully reduced to 2 unit cells from 4 unit cells.An uneven distribution of Mn²⁺,namely vertical electronic phase separation?VEPS?,was realized in the LaMnO₃ films with 1%compressive strain on La_0.3Sr_0.7Al_0.65Ta_0.35O₃?LSAT?substrates,leading to the vertical magnetic phase separation and unexpected exchange bias.The thickness dependent metal-to-insulator transition of NdNiO₃ films was investigated.As the results indicated,for films below the critical t hickness?t_c?for strain relaxation,the metal-to-insulator transitions temperature(T_MIT)was modulated by orbital polarization,as a preferential occupancy of x²-y² orbits would enhance the overlap of Ni 3d and O 2p orbits,thus stabilizing the metallic phase and reducing the metal-to-insulator transition temperature(T_MIT);for films with thickness above t_c,a change of T_MIT was due to strain relaxation.Attempts to modify the orbital polarization were also made.In LaNiO₃/BaTiO₃heterostructures,an electrically switchable and bi-direction control of the orbital polarization of LaNiO₃ films was achieved by the BaTiO₃ ferroelectric polarization with a range of up to 25%,which was achieved by the Ti ferroelectric displacement controlled Ti-O-Ni covalent bonding,providing an effective way to regulate the orbital occupancy of nicklate oxides.With careful investigation on the electronic configuration,strain state and orbital occupancy of high quality?La,Sr?MnO₃/LaNiO₃ heterostructures,charge transfer was proved to induce a preferred out-of-plane 3z²-r² orbital occupancy.Further,the compressive strain exerted by LaNiO₃ toplayer would promote out-of-plane 3z²-r² orbital occupancy,reducing in-plane magnetic coupling and promoting the formation of magnetization depression and spin glass.Furthermore,the spin glass,consisting of a seris of weak magnetic moments due to magnetization depression,was proved to be the origin of the novel exchange bias at the ferromagnetism/paramagnetism interface.