| Since the discovery of colossal magnetoresistance effect (CMR) in perovskite manganites, it has sparked considerable renewed interests in these long-known materials with an eye towards both an understanding of the CMR mechanism and related properties and potential applications in magnetic information storage and low-field mangetic sensors. Beside the CMR effect, these materials also exhibit intriguing physical properties such as insulator-metal transition and structure transition induced by applied magnetic-field or electric-field, charge/orbital/spin ordering, and phase separation etc. The full understanding of these properties will definitely stimulate the progress of condensed matter physics.In this thesis, the element-substitution effects, microcosmic electronic nematic order, nano-scaled domain wall pinning and their correlated effects are minutely studied, by both the measurements of macroscopical physical properties and the analysis of microstructure, in phase separated B-site Cu substituted LaMn1-xCuxO3 and the related A-site Ca substituted La1-yCayMn0.90Cu0.10O3 perovskites. A further understanding of nano-scaled magnetic domain (wall) structure, microcosmic electronic ordering/disordering transition and their correlated effects in magnetic/electrical transport properties is obtained. The whole thesis consists of six chapters.The first chapter aims at a brief overview of the history, progress, and current status of magnetoresistance effect. The abundant physical contents and potential applications in CMR manganites are also introduced, including crystal structure, electronic structure, magnetic/electrical transport mechanism, all kinds of ordering states, etc. By these illustrations, one may acquire a basic sight on magnetoresistance effect as well as magnetoelectronics. Meanwhile, the aims and motivations of the present work are synoptically given.In Chapter Two, the main methods and principles in our experiments are involved, including the preparation of samples, structural analysis and physical property measurements such as electrical transport, AC/DC magnetization, observation of crystal grains and their boundaries by electron microscopy, and so on.In Chapter Three, the effects of A-site and B-site substitution by foreign elements are studied by preparing LaMn1-xCuxO3 and La1-yCayMn0.90Cu0.10O3 samples. Firstly, the effects of nonmagnetic Cu substitution for Mn are studied in LaMn1-xCuxO3 system. The results confirm that the strong double-exchange ferromagnetism and large CMR effect can be achieved by directly Cu substitution into B-site. And when the Cu content is at 10%, both the double-exchange and CMR are optimal. Then, the effects of A-site Ca substitution for La are also studied at a fixed Cu substitution level x=0.10, especially the friction effects between differect phases or neighbouring domains/clusters, the large thermal hysteresis is also discussed. This part is helpful for the understanding of CMR mechanism and exploring new CMR materials.The fourth chapter is mainly about the study of metamagnetic step-like transition in La1-yCayMn0.90Cu0.10O3 manganites. A new theory of electron nematic ordering model is approved to be effective rather than phase separation, which is important for metamagnetism study and a further understanding of the phase separation model.In Chapter Five, the anomalous magnetic behavior below 100 K in dilute Cu substituted LaMn1-xCuxO3 system is studied systematically, and the domain wall pinning effect is confirmed by our measurements. The TEM pictures show that the critical size of single domain particle is at ablout RC ~ 100 nm. These results are useful to get to the bottom of the perplexing magnetic domain structure and build up a clear phase diagram at low temperatures in manganites.In Chapter Six, a summarization of the present work is given, altogether with the significance of this work.
|
PDF Full Text Request