  Sr$_2$RuO$_4$, an unconventional superconductor, is known to possess an
incommensurate spin density wave instability driven by Fermi surface nesting.
Here we report a static spin density wave ordering with a commensurate
propagation vector $q_c$ = (0.25 0.25 0) in Fe-doped Sr$_2$RuO$_4$, despite the
magnetic fluctuations persisting at the incommensurate wave vectors $q_{ic}$ =
(0.3 0.3 L) as in the parent compound. The latter feature is corroborated by
the first principles calculations, which show that Fe substitution barely
changes the nesting vector of the Fermi surface. These results suggest that in
addition to the known incommensurate magnetic instability, Sr$_2$RuO$_4$ is
also in proximity to a commensurate magnetic tendency that can be stabilized
via Fe doping.
