Strong anisotropy within a Heisenberg model in the J=1/2 insulating   state of Sr2Ir0.8Ru0.2O4
  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. 
S. Calder, J. W. Kim, A. E. Taylor, M. H. Upton, D. Casa, Guixin Cao,   M. D. Lumsden, A. D. Christianson
cond-mat.str-el cond-mat.mtrl-sci cond-mat.supr-con