  Resonant inelastic X-ray scattering (RIXS) at the $L$-edge of transition
metal elements is now commonly used to probe single magnon excitations. Here we
show that single magnon excitations can also be measured with RIXS at the
$K$-edge of the surrounding ligand atoms when the center heavy metal elements
have strong spin-orbit coupling. This is demonstrated with oxygen $K$-edge RIXS
experiments on the perovskite Sr$_2$IrO$_4$, where low energy peaks from single
magnon excitations were observed. This new application of RIXS has excellent
potential to be applied to a wide range of magnetic systems based on heavy
elements, for which the $L$-edge RIXS energy resolutions in the hard X-ray
region is usually poor.
