Doping Evolution of Magnetic Order and Magnetic Excitations in (Sr sub(1-x) La sub(x)) sub(3) Ir sub(2) O sub(7)

We use resonant elastic and inelastic x-ray scattering at the Ir-L sub(3) edge to study the doping-dependent magnetic order, magnetic excitations, and spin-orbit excitons in the electron-doped bilayer iridate (Sr sub(1-x) La sub(x)) sub(3) Ir sub(2) O sub(7)(0[< or =]x[< or =]0.065). With increasing doping x, the three-dimensional long range antiferromagnetic order is gradually suppressed and evolves into a three-dimensional short range order across the insulator-to-metal transition from x=0 to 0.05, followed by a transition to two-dimensional short range order between x=0.05 and 0.065. Because of the interactions between the J sub(eff)= [1/2] pseudospins and the emergent itinerant electrons, magnetic excitations undergo damping, anisotropic softening, and gap collapse, accompanied by weakly doping-dependent spin-orbit excitons. Therefore, we conclude that electron doping suppresses the magnetic anisotropy and interlayer couplings and drives (Sr sub(1-x) La sub(x))3Ir sub(2) O sub(7) into a correlated metallic state with two-dimensional short range antiferromagnetic order. Strong antiferromagnetic fluctuations of the J sub(eff)= [1/2] moments persist deep in this correlated metallic state, with the magnon gap strongly suppressed.