  Axion-photon conversion is a prime mechanism to detect axion-like particles
that share a coupling to the photon. We point out that in the vicinity of
neutron stars with strong magnetic fields, magnetars, the effective photon mass
receives comparable but opposite contributions from free electrons and the
radiation field. This leads to an energy-dependent resonance condition for
conversion that can be met for arbitrary light axions and leveraged when using
systems with detected radio component. Using the magnetar SGR J1745-2900 as an
exemplary source, we demonstrate that sensitivity to $|g_{a\gamma}| \sim
10^{-12}\,\rm{GeV^{-1}}$ or better can be gained for $m_a \lesssim 10^{-6}\,\rm
eV$, with the potential to improve current constraints on the axion-photon
coupling by more than one order of magnitude over a broad mass range. With
growing insights into the physical conditions of magnetospheres of magnetars,
the method hosts the potential to become a serious competitor to future
experiments such as ALPS-II and IAXO in the search for axion-like particles.
