Perovskite Manganite Epitaxial Thin Films And Heterojunction Preparation And Related Materials Research

The perovskite-type manganese oxides show a wide range of applications potential in magnetic storage, magnetic sensors, spin valve, spin transistors etc. because of colossal magnetoresistance (CMR) effect. Since most applications require thin film, it is of great importance to be able to prepare high quality single-crystal epitaxial films and have a full knowledge of all kinds of properties in them. Recently, due to the progress in preparation technique, the growth optimization of manganite thin films in nano-scale has become possible and some breakthroughs in the fabrication of related prototype devices have been obtained. La_0.67Sr_0.33MnO₃ (LSMO) is one of the most widely systems used in devices, and its high-quality epitaxial films are obviously expected. In addition, there is only negative magnetoresistance effect in manganese oxides in general, however, it is intriguing that not only negative MR but also positive MR, even a bias-dependent MR was observed in manganite-based heterojunctions.This dissertation mainly focuses on the strain relaxation in LSMO film and the modulation of La_1-xCa_xMnO₃/SNTO p-n junction in magnetic fields. In LSMO films, an in-plane modulated superlattice originating from shear strain relaxation was found; in La_1-xCa_xMnO₃/SNTO p-n junction, our results show that the complicated band structure of La_1-xCa_xMnO₃ plays a decisive role in the sign and magnitude of magnetoresistance, and on the basis of this study, the band structure evolution of La_1-xCa_xMnO₃ with magnetic field in energy scale is obtained. Moreover, the role of cooperative Jahn-Teller distortion in the ferromagnetic insulating behavior of Pr_0.7Ca_0.3MnO₃ is also studied by the ultrasonic attenuation measurement in magnetic field.This dissertation consists of five chapters. The arrangement of the chapters and the main contents in each chapter are presented as follows:In chapter 1, we make a brief introduction to all kinds of magnetoresistance effects; the basic properties of the perovskite-type manganese oxides, including research history, crystal structure, electronic structure, magnetic structure, and the electronic and magnetic phase diagrams; the strain relaxation of manganite thin films; and the research progress in application.In chapter 2, the strain relaxation and its effect on the in-plane superlattice formation of epitaxial LSMO thin films on the (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 (001) [LSAT (001)] substrates were investigated as a function of film thickness by X-ray diffraction. Rocking curves and reciprocal space mappings (RSM) around (002) and (003), (103) reflections reveal that the film with a thickness above 27 nm is almost fully relaxed and has a modulated superlattice structure. The recovery of the pseudocubic angle of LSMO resulting from the relaxation of the shear strain introduced by the growth of rhombohedral LSMO on cubic LSAT was suggested to be the origin of the superlattice structure.In chapter 3, the origin of the complex magnetoresistance in La_1-xCa_xMnO₃/SNTO p-n junction was studied by combining the complicated band structure of La_1-xCa_xMnO₃ with charge injection characteristics in p-n junction. Due to the subtle interplay among the Hund's coupling, JT effect and crystal field of La_1-xCa_xMnO₃, the filling of the electrons from SNTO in the different subbands of LCMO at the LCMO-SNTO interface plays a key ingredient in understanding the remarkable MR variations for the heterojunction. Our results also suggest that MASC and MISC are responsible for the negative and positive MR of the system, respectively.In chapter 4, the shift of the maximum (or minimum) position in unusual magnetoresistance (MR) vs V curves of the heterojunctions composed of La_1-xCa_xMnO₃ (LCMO, x=0.18 and 0.3) and 0.5 wt% Nb-doped SrTiO₃ (SNTO) was used to investigate the change of band gaps of LCMO with magnetic field.In chapter 5, the ultrasonic properties of Pr_0.7Ca_0.3MnO₃ under magnetic field were investigated and two sound velocity anomalies were observed. The first one is around 219 K, which is ascribed to the formation of charge ordering in 0T and to the ferromagentic spin fluctuation in 6T and 14T, respectively. The latter one takes place at the temperature lower than 150 K, which is ascribed to long-range orbital-ordering (OO). With increasing magnetic field, the magnitude of the relative change of sound velocity becomes smaller and elastic energy loss peak P₂ shifts towards lower temperature. These results illuminate that the cooperative JT distortion favors the establishment of the long-range OO and the FMI phase.