Magnetic excitations in the square-lattice iridate Ba2IrO4

We report a resonant inelastic x-ray scattering investigation of ultrathin epitaxial films of Ba2IrO4, and compare their low-energy magnetic and spin-orbit excitations to those of their sister compound Sr2IrO4. Due to the 180∘ Ir-O-Ir bond, the bandwidth of the magnon and spin orbiton is significantly larger in Ba2IrO4, making it difficult to describe these two types of excitations as separate well-defined quasiparticles. Both types of excitations are found to be quite sensitive to the effect of epitaxial strain. In addition, we find that the d-level inversion observed in Sr2IrO4 is absent in Ba2IrO4, as predicted in recent theoretical studies. Our results illustrate that the magnetic properties of Ba2IrO4 are substantially different from those of Sr2IrO4, suggesting that these materials need to be examined more carefully with electron itinerancy taken into account.