Structure And Properties Of La_0.8Sr_0.2MnO₃ Epitaxial Thin Films And Heterojunctions

Since the discovery of Giant magnetoresistance (GMR) by Albert Fert and Peter Griinberg in 1990's, the studies of it have become one of main topics in strongly correlated electron system due to its extensive and important application in industry, for example, computer hard disk, magnetic sensor, read-heads, magnetic memory material, etc, especially the discovery of GMR in perovskite manganese oxides materials whose GMR depending on thin film properties. These manganese oxides show paramagnetic-ferromagnetic phase transition, insulator-metal phase transition, phase separation and interplay between electron, spin and charge orders etc physical phenomenon of great significance in fundamental physics research; the studies and exploration of GMR epitaxial thin films offer an effective path to development and application of magnetic spin-valve, magnetic tunneling junction and heterojunction.The structure and electromagnetic properties of doped polycrystalline bulk and epitaxial thin films of La0.8Sr0.2MnO3 (LSMO) are systematically investigated in this thesis. Effects of preparation conditions and thickness on microstructure, transport properties and magnetism of films are carefully investigated respectively. Preparation and measurement methods of films and bulks samples are also introduced, placing emphasis on X-ray diffraction (XRD) and pulse laser deposition (PLD). In addition, thin films heterojunction La0.8Sr0.2MnO3/NSTO (LSMO/NSTO) are fabricated and the rectifying characteristics of LSMO/NSTO are also analyzed and discussed.The whole thesis consists of four chapters and the main contents of every chapter are as follows:Chapter one:The physical properties, research and application of GMR materials, including bulk, epitaxial thin films and heterojunction are introduced and summarized. Firstly, the history, application value and physical mechanism of GMR are reviewed and the crystal structure, electrical properties, magnetic structure and transport properties of perovskite manganese oxides, one of the GMR materials, are expatiated. Secondly, the difference of properties between thin films and bulks whose consist of same materials are pointed out. Lastly, we introduce the application and properties of LSMO/NSTO heterojunction and make a prospect of further application.Chapter two:La0.8Sr0.2MnO3 epitaxial thin films with different thickness are fabricated and the representing means and measurement methods towards bulks and films are introduced. Firstly, the preparation of bulks and fabricating technologies of thin films are reviewed, especially PLD and fabricating thin films process. Secondly, representing means, including XRD, reciprocal space maps, scanning tunneling microscope and atomic force microscope are introduced. Lastly, the instruments for measuring the resistivity and magnetism of films are simply described respectively.Chapter three:the influence of doping content and lattice mismatch on thin films' crystal growth and microstructure and the effect of thickness on thin films'structure, transport properties and magnetism are investigated in detail. The result of XRD manifest that doping content and thickness nearly don't influence the microstructure of La1-xSrxMnO3 bulks and that those LSMO/STO thin films with different thickness possess of high crystalline and epitaxial quality. Measurements indicate that the resistivity decreases and the insulator-metal transition temperature (Tp) increases with increasing of the film thickness. The Tp will remain a fixed value which is the Tp of bulk of LSMO when LSMO films thickness reaches a critical value. Magnetic measurements make it clear that LSMO thin films'magnetization is far greater than vertical when applied magnetic field is parallel with film surface and the magnetization reduces with the increase of thickness.Chapter four:firstly, the development history, present research and application of magnetite-based p-n junction are reviewed. Secondly, the process of fabricating La0.8Sr0.2MnO3/NSTO thin films junction is introduced. Last, thickness effects on the rectifying characteristics of LSMO/NSTO are discussed coming to a conclusion that the rectifying characteristics of LSMO/NSTO are getting better for thicker junction.