Studies Of Magnetic And Related Properties In Charge Ordering System Of Perovskite Manganite Oxides

As we know, the studies of perovskite manganite oxides have attracted much renewed attention in strongly correlated electron system since the discovery of colossal magnetoresistance(CMR) effect. Because CMR effects are great valuable in industrial demand, e. g. the magnetic memory, magnetic random access memories, magnetic sensors and magnetic refrigeration; furthermore, as a fundamental physic research, this strongly correlated electron system exhibits many intriguing physical properties, e. g. paramagnetic-ferromagnetic phase transition together with insulator-metal transition, charge ordering, orbital ordering, phase separation, Jahn-Teller distortion and the coupling between them; the investigation in the manganites and the understanding of CMR mechanism not only have important significancy for the spintronic application, but also stimulate the great progress in the whole condensed matter physics.In this thesis, the charge ordering manganites were investigated using doubly doping method to clarify its mechanism. Specially, the observed instability in this sysytem is also studied in details.The whole thesis consists of six chapters.Chapter 1: We give a general review of the history and the present research situation in perovskite manganite oxides. Some related properties, such as the structural, magnetic, electronic transport, CMR effect, charge/orbital ordering, phase separation and theoretical model, are introduced. In the end, we sum up the current research and some existent issues.Chapter 2: The instability in the half -doping manganites is discussed. It seems that the short-ranged charge ordering phase plays a key role in the presence of instability because most of reports always focus on this system with Mn-site doping. However, we think that the intrinsical issue lies in strain effect rather than the short-ranged charge ordering phase. Therefore, we investigate the long-ranged charge ordering properties in Nd_0.5Sr_0.45Ca_0.05MnO₃ system. The occurrence of Ca ions does not influence the formation of long-ranged charge ordering but induces a slight A-site disorder as well as phase separation in this system. Upon the thermal cycling process, the successive decrease of magnetization and the enhancement of resistance are observed, exhibiting a similar instability in short-ranged CO system. The systematic phase competition causes the present of strain effect in their interfaces, which is the intrinsic reason for instability in the CO system.Chapter 3: The effect of Ga doping in Nd_0.55Sr_0.45MnO₃ on magnetic and transport properties is investigated. Upon Ga substitution, no structure change is found. However, it induces the B-site disorder. At a relevant doping region, the phenomenon of Griffiths phase is observed due to an appearance of inhomogeneous distribution of...