Magnetic structure and dynamics of Rh-doped Sr$_2$IrO$_4$ probed by resonant x-ray scattering

The physics of 5d iridates has recently attracted considerable attention due to the potential for novel electronic and magnetic ground states driven by strong spin-orbit coupling (SOC). One material which has attracted particular interest is the layered perovskite Sr$_2$IrO$_4$, which has been proposed as the first experimental realization of a spin-orbital Mott insulator with a j$_{eff}$ = 1/2 ground state [1,2]. It has been shown that by substituting Ir$^{4+}$ (5d$^5$) ions for Rh$^{4+}$ (4d$^5$), the strength of the SOC in this system can be tuned through a series of electronic phase transitions [3]. We have performed resonant magnetic x-ray scattering (RMXS) and resonant inelastic x-ray scattering (RIXS) measurements to determine the effect of Rh-doping on the magnetic structure and excitation spectrum of Sr$_2$Ir$_{1-x}$Rh$_x$O$_4$. We find that increasing Rh concentration results in (i) suppression of the magnetic transition temperature, (ii) a doping-induced change in magnetic structure, (iii) alteration of the magnon dispersion relation, and (iv) significant reduction of magnon lifetimes. [1] B.J. Kim et al, PRL 101, 076402 (2008). [2] B.J. Kim et al, Science 323, 1329 (2009). [3] T.F. Qi et al, PRB 86, 125105 (2012).