Generation And Physical Property Research Of Perovskite Manganese Oxides Films

The studies of perovskite manganese oxides have attracted much renewed attention since the discovery of colossal magnetoresistance (CMR) effect in the mid-1990s. Because CMR effects are great valuable in industrial demand, such as the magnetic memory, magnetic random access memories, magnetic sensors; however, these demands mainly depend on the properties of the thin-films. As a fundamental physics research, the manganite system exhibits many intriguing physical behaviors; e.g. paramagnetic-ferromagnetic phase transition together with insulator-metal transition, charge-orbital ordering, phase separation, Jahn-Teller distortion, double exchange model and the coupling between them; especially, the investigation in the manganite films not only have important significance for the spintronic application, but also stimulate the great progress in the growth of epitaxial thin films for all oxide or oxide-metal device.In this thesis, the influence of the lattice-misfit oxygen and strain contents on the thickness effects of CMR manganite thin films was carefully investigated. Specially, the in-situ deposition oxygen pressure affects on the structural and transport properties of the ultra-thin films is studied in details. The whole thesis consists of four chapters.Chapter 1: The general review of the history and present research situation of the perovskite manganite oxides physics and thin films is given, such as the crystal and electronic structure, double exchange model, charge-ordering, electronic transport, CMR effect, phase separation, effects of doping level and so on, are introduced. In the end, we sum up the current research in the manganite thin films and some existent issues.Chapter 2: The oxygen content effects on the structural and physical properties of the CMR films are discussed in epitaxial Nd_0.7Sr_0.3MnO₃ (NSMO) thin films. it is indicated that the structural and transport properties of ultra-thin films strongly depend on the deposition oxygen pressures; but for the partially relaxed thicker films, the structure and transport behaviors is affected by both in-situ deposition and the post-annealing process, due to the column boundaries in thicker films could take-up a atomic oxygen from the ex-situ oxygen annealed. To get a higher electrical conductivity in the ultra-thin films, a higher deposition oxygen pressure is crucial. For the thicker NSMO films, expecialy the vacuum processing is observed.Chapter 3: Effects of the lattice-misfit strain induced from the substrate on the electro-magnetic properties of the NSMO thin films was studied. It was found that the strain state of the NSMO films is closely related to the transport behaviors. The structural and transport properties of the thin strained films depend strongly on the Jahn-Teller term of lattice distortion and the thickness. For the thicker NSMO films, the large is the lattice misfit between the film and substrate, the small is the strain relaxation thickness, and the metal-insulator transition temperature T_P rapidly approaches that T_P of the bulk materials with increasing of the film thickness. So, the biaxial strain is an important factor for the thickness effect in epitaxial manganite films, and it should be considered for fabricating a high quality of CMR films.Chapter4: Thickness dependent variation on the resistivity in epitaxial La_0.7Sr_0.3MnO₃(LSMO) thin films Epitaxial La0.7Sr0.3MnO3 single crystal films of different thickness values deposited on cubic LaAlO3(001) single crystal substrates, and x-ray diffraction spectrum shows that LSMO epitaxial films have properties which is from partially strained to fully relaxed with film thickness increasing on the same substrate. Resistivity measurement shows that the relaxed film has higher resistivity than that of the strained film, because the relaxed film contains the high deficiency density. In this paper, however, the relaxed property of the same thickness LSMO epitaxial film grown on the different substrates is also discussed.