Strong anisotropy within a Heisenberg model in the Jeff=12 insulating state of Sr2Ir0.8Ru0.2O4

The dispersive magnetic excitations in ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ have previously been well described within an isospin-1/2 Heisenberg model on a square lattice that revealed parallels with ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$. Here we investigate the inelastic spectra of ${\mathrm{Sr}}_{2}{\mathrm{Ir}}_{0.8}{\mathrm{Ru}}_{0.2}{\mathrm{O}}_{4}$ with resonant inelastic x-ray scattering (RIXS) at the Ir ${L}_{3}$ edge. The results are well described using linear spin-wave theory within a similar Heisenberg model applicable to ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$; however, the disorder induced by the substitution of $20%\phantom{\rule{4pt}{0ex}}{\mathrm{Ir}}^{4+}$ ions for ${\mathrm{Ru}}^{4+}$ removes longer range exchange interactions. A large spin gap (40 meV) is measured indicating strong anisotropy from spin-orbit coupling that is manifest due to the altered magnetic structure in ${\mathrm{Sr}}_{2}{\mathrm{Ir}}_{0.8}{\mathrm{Ru}}_{0.2}{\mathrm{O}}_{4}$ with $c$-axis aligned moments compared to the basal plane moments in the parent. Collectively the results indicate the robustness of a Heisenberg model description even when the magnetic structure is altered and the ${J}_{\mathrm{eff}}=1/2$ moments are diluted.