Study On Phase Separation In Electron-doped Manganites

Manganites have attracted much research attention in recent years due to the discovery of colossal magnetoresistance (CMR), which has the potential applications in spintronics, and the related rich physics. Manganites belong to the strongly correlated electron systems, which have the strong interactions of electron-lattice and electron-electron. These oxides show complex and interesting electronic and magnetic behavior, such as metal to insulator (M-I) transition, charge ordering (CO), orbital ordering (OO), and magnetic ordering.In recent years, the evidence of inhomogeneities in manganites is very strong, both in theory and experiments. In manganites the inhomogeneities (or phase separation) arise from phase competition between ferromagnetic metal (FMM) and antiferromagnetic insulator (AFI) phases. The reported length scale of inhomogeneity varies from a few nanometers to microns. One broad viewpoint holds that quenched disorder and long-range strain could have a large effect on the phase separation. It is believed now that the nature of the CMR effect is closely related to the phase separation and the phase transition can be as a result of the percolation process. In this field, much research activity is focused on hole-doped manganites, e.g. La1-xSrxMnO3 and La1-xCaxMnO3 (x<0.5). Currently, there has been some interest in studying the properties of electron-doped manganites because both electron-doped manganites and hole-doped manganites may open up very interesting applications in the emerging field of spintronics. The studies on electron-doped manganites are significative to enrich the physical properties of the phase-separated manganites, and further shed light on understanding the CMR mechanism.In the dissertation, we study the phase separation in the electron-doped manganites, including A-site and B-site doping systems, by means of structural, magnetic, and electrical properties measurements. In addition, mechanical energy dissipation spectra (internal friction) experiments are applied to study the phase separation behavior. Our results show that internal friction measurement technique is a useful tool for studying the phase separation behavior in CMR materials.The dissertation is divided into five chapters, and the main content of each chapter is summarized as follows:In the first chapter, we briefly review the research history of magnetoresistance and the related physical properties of CMR manganites. Manganites show some interesting properties including M-I transition, charge ordering, orbital ordering, and phase separation. Recent research indicates that manganites are intrinsically inhomogeneous. The intrinsic inhomogeneities play a fundamental role in the CMR effect of manganites.In chapter two, we systemically study the structural, magnetic, and electrical properties of A-site doped Sr1-xCexMnO3 and Ca1-xCexMnO3. The experimental results show that the antiferromagnetic insulator and ferromagnetic metal phases coexist in the doped samples, which is due to the A-site doping disorder effect. Ce substitution for Sr or Ca induces the A-site ion disorder distributing and enhances the fluctuation of AFI and FMM phases. We also observed electroresistance and nonlinear conduction in Ca0.9Ce0.1MnO3 sample. The origin of these phenomena is discussed in view of current induced collapse of CO state associated with phase separation mechanism.In chapter three, we study the structural, magnetic, electrical, and thermal properties of B-site doped LaMn_1-xMo_xO₃ and Ca₄Mn_3-xMo_xMnO₁₀. It shows that small content Mo doping can change the ground state properties. At low temperatures, the doped samples show the coexistence of FM and AFM phases. Compared with A-site doping, B-site doping can induce phase separation more easierly.In chapter four, we study the internal friction of the electron-doped Sr0.8Ce0.2MnO3 and Bi0.4Ca0.6MnO3. The internal friction peak in paramagnetic (PM) region of Sr_0.8Ce_0.2MnO₃ sample is observed, which is related to the formation of FM clusters in PM region. Below the CO transition temperature TCO, Bi_0.4Ca_0.6MnO₃ sample shows the coexistence of CO and charge disordering (CD) phases, which is induced by inhomogeneous strain. The observed internal friction peak related to a relaxation mechanism could be ascribed to the motion of CO and CD domains.Finally the brief summary and prospect of this dissertation is given in chapter five.