  The dispersive magnetic excitations in Sr2IrO4 have previously been well
described within an isospin-1/2 Heisenberg model on a square lattice that
revealed parallels with La2CuO4. Here we investigate the inelastic spectra of
Sr2Ir0.8Ru0.2O4 with resonant inelastic x-ray scattering (RIXS) at the Ir
L3-edge. The results are well described using linear spin-wave theory within a
similar Heisenberg model applicable to Sr2IrO4, however the disorder induced by
the substitution of 20% Ir4+ ions for Ru4+ 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 Sr2Ir0.8Ru0.2O4 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=1/2 moments diluted.
