Studies Of Frustrated Magnetism And Electrical Properties Of Pyrochlore Iridium Oxides

In recent years, people found that the spin-orbit coupling might lead to a new physical form of substance:topological insulators. Topological insulator has become a fertile ground for studies of physics driven by strong spin-orbit interactions.5d transition metal oxides provide a fascinating system to study the interplay ang competition between electron correlation and spin-orbit coupling. Among all the 5d transition metal oxides, iridate compounds forming in the pyrochlore lattice have attracted particular interest as they are predicted to be one of the candidates to study electron correlation, spin-orbit interaction and the potential topological phases. The pyrochlore iridium oxides A2Ir2O7(A=Y、Bi or lanthanide element) belong to cubic symmetry and Fd-3m space ground. Early experimental work found that with the increasing ionic radius of A the series goes from magenetic insulators to unconventional metallic systems without magnetic ordering. Bi2IrO7 offers the opportunity to separate the interesting properties due to the rare-earth and iridium ions, a compound without a rare-earth magnetic moment or f electron, and has a different ion size to yttrium.In this dissertation, we briefly introduce the physical properties and the history and progress of pyrochlore iridium oxides, firstly. Then, three pyrochlore iridates of different A-site element Y、Eu and Bi are as research objects, by studying their electrical transport properties, lattice structure, magnetism and thermodynamic properties. In addition, we focus on polycrystal Bi2Ir2O7, and introduce in detail spin-orbit coupling, frustration effection, electrical transport properties and magnetism by doping Ca and Mn ions in Bi sites and Ir sites, respectively. The main results are as following:We carry out a study to compare the structure, magnetization, electrical transport properties and heat capacity of the pyrochlore iridates A2Ir2O7 (A=Y, Eu, Bi). It is found that magnetic frustration and long-range magnetic order coexist in Y2Ir2O7 and Eu2Ir2O7, while Bi2Ir2O7 shows magnetic frustration or short-range magnetic order without long-range magnetic order. The variations of the radius of A-site ion affect the conduction bandwidth, correlation and Ir-Ir exchange energy, both of them in turn determine transport properties and ground-state configuration. We also find that the antiferromagnetic ordering of pyrochlore iridates is weakly frustrated. The anisotropy based on spin-orbit coupling plays a key role in releasing the frustration.We successfully composed Ca and Mn doping polycrystalline samples. It is found that the index compound BiIrO7 perform magnetic instability and metallic phase. When substitute Bi ions by Ca ions with smaller ionic radius, the phenomenon of metal - insulator transformation appears in this system and the transformation temperature Tc increases with the increase of Ca ion doping. We do not obtain magnetic phase transition at the appropriate metal-insulator transition temperature Tc, hence the reason of the metal-insulator transition is due to the enhancement of electron scatter and the increasing disorder of crystal lattice induced by Ca doping. In addition, Ca doping decreases the angle of Ir-O-Ir and the hybridization between Ir 5d and Bi 6p electrons, resulting in the enhancement of antiferromagnetism and variation of magnetic frustration shown by analysises and calculations. When substitute Ir ions by 3d5 Mn ions, metal-insulator transition is observed, yet. There are two conceivable reasons for this metal-insulator transition induced by Mn doping. On the one hand, the lattice disorder is enhanced by Mn dopig. On the other hand,3d elements have smaller ionic radius, narrower band gap and its electronic shielding effect is smaller, the coulomb interaction between electrons is bigger, thus, when we use Mn ions instead Ir ions the bond angle of Ir-O-Ir turns smaller. We do not detect any magnetic phase transition induced by Mn doping, but from the fitting results of Curie-Weiss law, the antiferromagnetism of the system is increaed with the increment of Mn content. The magnetoresistance of doped samples decreases with the increase of magnetic field, in contrast to the variation trend of parent material. The emergence of this phenomenon is related the new interations of Mn-Mn and Mn-Ir exist in Mn doping samples.