Experime Ntal Studies On Isoelectronic Doping Of AFe₂As₂(A=Eu,Ba) Systems

Since its discovery, superconductivity has aroused wide attention in phys ics, and is all the while one of the most popular subjects. The discovery of iron-based superconductors in the year of 2008 is another important breakthrough after the copper-oxide superconductivity in high temperature superconductivity, which brought great chance and challenge for the superconductive study. The crystal structure and chemical composition of the layers are crucial and sensitive for the emerge of supe rconductivity.Based on the earlier study work, my dissertation focuses on the iso-electronic doping effects on the EuFe2As2 and BaFe2As2 systems. EuFe2As2 and BaFe2As2 possess the same ThCr2Si2-type structure, however, the structural and magnetic transition happen at 200K and 140K respectively. By iso-electronic doping methods, some important results and electronic phase diagrams are obtained.My dissertation includes four chapters. The first chapter is the introduction, including the process of superconductivity development, two basic properties of superconductors, the solid magnetism and so oa The second chapter summarizes the experimental research progress of iron-based superconductivity, especially on the 1111 and 122 systems.In the third chapter, we demonstrate the results on P doping at As site in EuFe2As2. A series of EuFe2(As_1-xP_x)₂ polycrystalline were synthesized, and powder X diffraction and resistivity measurement were performed. The isovalent substitution of As with P leads to the shrinkage of the lattice and generates chemical pressure. The phase diagram is obtained, which shows that P doping at As site suppress the SDW transition and induces superconductivity. In the area of 0.2≤x≤0.4, superconductivity emerges, and coexists with ferromagnetism. Ferromagnetic transition happe ns in the area of x>0.4.The fourth chapter introduces the work on iso-electronic Fe-site doping with Cr and Ni in Ba(Fe_1-xCr_x/2Ni_x/2)₂As₂. We performed some property characteristics and obtained phase diagram. We found that the Cr and Ni doping induces ferromagnetic interreaction. Doping suppresses the SDW transition quickly and spin glass state emerge. With increasing the doping level, spin glass state evolves into ferro magnetism. BaCrNiAs2 shows a ferromagnetic transition at 64K.