We study the evolution of magnetic excitations in the disordered two-dimensional antiferromagnet ${\mathrm{Sr}}_2{\mathrm{Ir}}_{1-x}{\mathrm{Ru}}_x{\mathrm{O}}_4$. The maximum energy of the magnetic excitation remains robust up to $x=0.77$, with a gap opening at low dopings and increasing to over 150 meV at $x=0.77$. At these higher Ru concentrations, the dispersive magnetic excitations in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ are rendered essentially momentum independent. Up to a Ru concentration of $x=0.77$, both experiments and first-principles calculations show the Ir $J_{\text{eff}}=1/2$ state remains intact. The magnetic gap arises from the local interaction anisotropy in the proximity of the Ru disorder. Under the coherent potential approximation, we reproduce the experimental magnetic excitations using the disordered Heisenberg antiferromagnetic model with suppressed next-nearest-neighbor ferromagnetic coupling.

• Received 10 August 2016
• Revised 25 January 2017

DOI:https://doi.org/10.1103/PhysRevB.95.121103