A Systematic Study of the Effect of Spin-Orbit Interaction on Properties of Tetravalent and Pentavalent Iridate Compounds

Previous studies of iridates have shown that an interplay of strong SOI, Coulomb interaction U, Hund’s rule coupling and crystalline electric fields result in unexpected insulating states with complex magnetic states. The novel Jeff =1/2 insulating state first observed in Sr2IrO4 is a direct consequence of such an intriguing interplay and is one of the central foci of this dissertation study. The work presented here consists of three projects: (1) Effects of Tb doping on Sr2IrO4 having tetravalent Ir4+(5d5) ions; (2) Emergence of unexpected magnetic states in double-perovskite (Ba1-xSrx)2YIrO6 with pentavalent Ir5+(5d4) ions in the presence of strong SOI, and (3) The coexistence of a charge and magnetic order in a magnetic dimer chain system, Ba5AlIr2O11, which has both tetravalent Ir4+ (5d5) and pentavalent Ir5+ (5d4) ions. A significant portion of this dissertation will focus on Tb doped Sr2IrO4. A central finding of this work is that slight Tb doping (3%) readily suppresses the antiferromagnetic state but retains the insulating state, indicating an unusual correlation between the magnetic and insulating states as a result of the presence of the strong SOI. However, SOI is not the only significant phenomenon. The study on the double-perovskite (Ba1-xSrx)2YIrO6 revealed an exotic magnetic ground state, in sharp contrast to the anticipated singlet ground state in the strong SOI limit, raising an urgent question: is SOI as dominant as was initially anticipated in the iridates? Finally, this study turns to a system containing both Ir4+ and Ir5+ ions, Ba5AlIr2O11. This system features dimer chains of two inequivalent octahedra occupied by tetravalent Ir4+ (5d5) and pentavalent Ir5+ (5d4) ions respectively. Ba5AlIr2O11 undergoes charge and magnetic order transitions at 210 K and 4.5 K, respectively. SOI-driven physics is a rapidly evolving field with an ever growing list of theoretical proposals which have enjoyed very limited experimental confirmation thus far. This study has revealed a large range of interesting phenomena in the iridates that defy conventional theoretical arguments and that help to fill an experimental void in this field

Electrical Control of Structural and Physical Properties via Strong Spin-Orbit Interactions in Sr\u3csub\u3e2\u3c/sub\u3eIrO\u3csub\u3e4\u3c/sub\u3e

Electrical control of structural and physical properties is a long-sought, but elusive goal of contemporary science and technology. We demonstrate that a combination of strong spin-orbit interactions (SOI) and a canted antiferromagnetic Mott state is sufficient to attain that goal. The antiferromagnetic insulator Sr2IrO4 provides a model system in which strong SOI lock canted Ir magnetic moments to IrO6 octahedra, causing them to rigidly rotate together. A novel coupling between an applied electrical current and the canting angle reduces the Néel temperature and drives a large, nonlinear lattice expansion that closely tracks the magnetization, increases the electron mobility, and precipitates a unique resistive switching effect. Our observations open new avenues for understanding fundamental physics driven by strong SOI in condensed matter, and provide a new paradigm for functional materials and devices

Evidence for a Low-Temperature Magnetic Ground State in Double-Perovskite Iridates with Ir\u3csup\u3e5+\u3c/sup\u3e(5\u3cem\u3ed\u3c/em\u3e\u3csup\u3e4\u3c/sup\u3e) Ions

We report an unusual magnetic ground state in single-crystal, double-perovskite Ba2YIrO6 and Sr-doped Ba2YIrO6 with Ir5+(5d4) ions. Long-range magnetic order below 1.7 K is confirmed by dc magnetization, ac magnetic susceptibility, and heat-capacity measurements. The observed magnetic order is extraordinarily delicate and cannot be explained in terms of either a low-spin S = 1 state, or a singlet Jeff = 0 state imposed by the spin-orbit interactions (SOI). Alternatively, the magnetic ground state appears consistent with a SOI that competes with comparable Hund\u27s rule coupling and inherently large electron hopping, which cannot stabilize the singlet Jeff = 0 ground state. However, this picture is controversial, and conflicting magnetic behavior for these materials is reported in both experimental and theoretical studies, which highlights the intricate interplay of interactions that determine the ground state of materials with strong SOI

Evidence for a Low-Temperature Magnetic Ground State in Double-Perovskite Iridates with Ir\u3csup\u3e5+\u3c/sup\u3e(5\u3cem\u3ed\u3c/em\u3e\u3csup\u3e4\u3c/sup\u3e) Ions

We report an unusual magnetic ground state in single-crystal, double-perovskite Ba2YIrO6 and Sr-doped Ba2YIrO6 with Ir5+(5d4) ions. Long-range magnetic order below 1.7 K is confirmed by dc magnetization, ac magnetic susceptibility, and heat-capacity measurements. The observed magnetic order is extraordinarily delicate and cannot be explained in terms of either a low-spin S = 1 state, or a singlet Jeff = 0 state imposed by the spin-orbit interactions (SOI). Alternatively, the magnetic ground state appears consistent with a SOI that competes with comparable Hund\u27s rule coupling and inherently large electron hopping, which cannot stabilize the singlet Jeff = 0 ground state. However, this picture is controversial, and conflicting magnetic behavior for these materials is reported in both experimental and theoretical studies, which highlights the intricate interplay of interactions that determine the ground state of materials with strong SOI