Investigation On Physical Properties And B-Site Cation Ordering Of Cr Doped YNnO₃

Hexagonal YMnO3as one of the typical multiferroic materials has drawn much attention in recent years. Because its multiferroicity is the coexistence of antiferromagnetism and ferroelectricity, it is insensitive to applied magnetic fields, which is an obvious disadvantage in potential applications. In order to enhance the ferromagnetic component in YMnO3, elemental substitutions on the Mn site have been widely studied to introduce ferromagnetic double exchange interaction or tilting antiferromagnetic state. Because insulating state is a prerequisite for ferroelectricity in compounds, however, we considered that double exchange mediated by the itinerant eg electrons goes against electric polarization in systems. Therefore, other mechanisms should be taken into account. Ferrimagnetism is one of the possible choices and is the research emphasis in this dissertation, because it not only increases magnetization but also maintains antiferromagnetic order and localizaiton of eg electron in systems.Remaining hole carrier density r=0.5unchanged, we focused on Y0.5+ySr0.5-yMn1-yCryO3(0≤y≤0.5) series, investigating the evolution of crystal structure, magnetic and transport properties in the double-doped manganites. Particularly for the half Cr-doped YMn0.5Cr0.5O3, we carried out detailed studies on its ferrimagnetism and possible double perovskite structure by using experimental and theoretical methods, respectively. There are five chapters in this dissertation, and each chapter is presented as fellow:In Chapter1, we first provide a brief review on the research history of manganite, and give a brief introduction to its crystal structure, magnetic structure, exchange interactions, transport properties, ordering effect, etc. Then we also give a brief introduction to the density funtional theory, and finally we describe the basic properties of hexagonal manganite YMnO3.In Chapter2, we first introduce the preparation method of the polycrystalline samples in this work, and then give a detailed description on the X-ray diffraction analysis method. Also, give a brief explanation of electrical transport and magnetic properties measurements. The last is a brief introduction to the Material Studio software and Castep package used in this dissertation.In Chapter3, we first investigate the evolution of structure in double-doped manganites polycrystalline samples Y0.5+ySr0.5-yMn1-yCryO3(0≤y≤0.5), and the results shows a transformation from a pseudo-cubic perovskite structure to a pseudo-tetragonal structure with the increase of y, accompanied by a gradual process of Mn/Cr ions layered ordering in the B site. The temperature dependence of magnetic susceptibility shows that the magnetic state at low temperatures evolves from an antiferromagnetic-spin-glass state for y=0, undergoing a ferromagnetic-spin-glass state, finally to a relatively stable ferrimagnetic state for y=0.5. The electrical transport properties of all samples show an insulator behavior dominated by the small polaron hopping mechanism and the activation energy increases significantly along with y. Consequently, the enhancement of ferromagnetic component in the Y0.5+ySr0.5-yMn1-yCryO3series may be attributed to the anisotropic ferromagnetic superexchange interactions of Mn3+ions collaborating with the ordered arrangement of Mn3+/Cr3+ions, rather than the double exchange interaction via eg-electron hopping process.In Chapter4, we make a detailed analysis on the ferrimagnetism and possible double perovskite structure of the half Cr-doped YMn0.5Cr0.5O3polycrystalline. We first fit its magnetic data by ferrimagnetic molecular field theory, and the results show that the system is made up of two ferromagnetic Mn3+/Cr3+sublattices with antiferromagnetic coupling between each other. While the magnetic coupling within Mn3+sublattice is ferromagnetic, the coupling within Cr3+sublattice is antiferromagnetic. Due to the collaboration of strong antiferromagnetic coupling between Mn3+/Cr3+sublattices and bigger ferromagnetic Mn3+sublattice, the Cr3+moments are aligned in a ferromagnetic order, and then the system enters ferrimagnetic phase at lower temperatures. The room temperature X-ray diffraction data of YCr0.5Mn0.5O3is analysed by the Rietveld refinement using B site disordered and rock salt/columnar/layered ordered models, respectively. The fitting results show no significant difference to indentify the ordered or disordered pattern, the reason may be that the Mn3+and Cr33+ions have similar ionic radius and electronic configuration, and thus X-ray diffraction is not sensitive enough to exactly distinguish Mn3+and Cr3+ions. Future investigation with the help of other research methods is still needed,In Chapter5, by employing the first-principles electronic structure calculations, we seek for the possible double perovskite structure of Y2MnCrO6. First of all, we build three model structures with Mn3+/Cr3+ions ordered in rock salt, columnar and layered patterns, and calculate and compare their total energy. Our results show that the ground state of Y2MnCrO6with the lowest energy is ferrimagnetic insulator with Mn3+/Cr3+layers alternatively stacking along the [001] direction, which is in good agreement with the experimental results. Finally, we give a reasonable explanation for the ferrimagnetism of Y2MnCrO6in the framework of semicovalent exchange mechanism proposed by J. B. Goodenough.