Combined Transport, Magnetization and Neutron Scattering Study of Correlated Iridates and Iron Pnictide Superconductors
The work performed within this thesis is divided into two parts, each focusing primarily on the study of magnetic phase behavior using neutron scattering techniques. In first part, I present transport, magnetization, and neutron scattering studies of materials within the iridium oxide-based Ruddelsden-Popper series [Srn+1IrnO3n+1] compounds Sr3Ir2O7 (n=2) and Sr2IrO4 (n=1). This includes a comprehensive study of the doped bilayer system Sr3(Ir1-xRux )2O7. In second part, I present my studies of the effect of uniaxial pressure on magnetic and structural phase behavior of the iron-based high temperature superconductor Ba(Fe1-xCox)2As2. Iridium-based 5d transition metal oxides host rather unusual electronic/magnetic ground states due to strong interplay between electronic correlation, lattice structure and spin-orbit effects. Out of the many oxides containing iridium, the Ruddelsden-Popper series [Srn+1IrnO3n+1] oxides are some of the most interesting systems to study both from the point of view of physics as well as from potential applications. My work is focused on two members of this series Sr3Ir2O7 (n=2) and Sr2IrO4 (n=1). In particular, our combined transport, magnetization and neutron scattering studies of Sr3Ir2O7 (n=2) showed that this system exhibits a complex coupling between charge transport and magnetism. The spin magnetic moments form a G-type antiferromagnetic structure with moments oriented along the c-axis, with an ordered moment of 0.35±0.06 µB/Ir. I also performed experiments doping holes in this bilayer Sr3(Ir1-xRux)2O7 system in order to study the role of electronic correlation in these materials. Our results show that the ruthenium-doped holes remain localized within the Jeff=1/2 Mott insulating background of Sr3Ir2O7, suggestive of `Mott blocking' and the presence of strong electronic correlation in these materials. Antiferromagnetic order however survives deep into the metallic regime with the same ordering q-vector, suggesting an intricate interplay between residual AF correlations in the Jeff=1/2 state and metallic nanoscale hole regions. Our results lead us to propose an electronic/magnetic phase diagram for Sr3(Ir1-xRux)2O7 system showing how the system moves from Jeff=1/2 antiferromagnetic Mott insulator (Sr3Ir2O7) to paramagnetic Fermi liquid metal (Sr3Ru2O7). On the other hand, our neutron scattering measurements on Sr2IrO4 (n=1), a prototypical Jeff=1/2 Mott insulator, showed that the spins arranged antiferromagnetically in ab-plane with an ordered moment comparable to that of Sr3Ir2O7. The second part of my work is comprised of a neutron scattering-based study of the Ba(Fe1-xCox)2As2 system, a bilayer family of iron-based high temperature superconductors. Undoped, this system exhibits either simultaneous or nearly simultaneous magnetic and structural phase transitions from a high temperature paramagnetic tetragonal phase to low temperature orthorhombic antiferromagnetic phase. With the gradual suppression of these two temperatures, the superconducting phase appears with the highest TC obtained just beyond their complete suppression. It has been proposed that these coupled magnetostructural transitions are secondary manifestations which arise as a consequence of electronic nematic ordering that occurs at a temperature higher than either of them. My work is mainly focused on probing the spin behaviors coupling to this electronic nematic phase. I devised a small device to apply uniaxial pressure along an in-plane high symmetry axis and studied the magnetic and structural behavior in series of Ba(Fe1-xCox)2As2 compounds via neutron scattering in presence of uniaxial pressure. There is an upward thermal shift in the onset of structural and magnetic transition temperature caused by this uniaxial pressure which is surprisingly insensitive to cobalt concentration in the absolute scale. Furthermore, on the first order side of the phase diagram (below the tricritical point), the structural and magnetic transitions are decoupled with magnetic transition following structural distortion. This study suggests the importance of both spin-lattice and orbital-lattice interactions in these families of compounds.