  Resonant elastic X-ray scattering (REXS) at the Eu $M_5$ edge reveals an
antiferromagnetic structure in layered EuCd$_2$Sb$_2$ at temperatures below
$T_\textrm{N}$ = 7.4 K with a magnetic propagation vector of $(0,0,1/2)$ and
spins in the basal plane. Magneto-transport and REXS measurements with an
in-plane magnetic field show that features in the magnetoresistance are
correlated with changes in the magnetic structure induced by the field. Ab
initio electronic structure calculations predict that the observed spin
structure gives rise to a gapped Dirac point close to the Fermi level with a
gap of $\Delta E \sim$0.01 eV. The results of this study indicate that the Eu
spins are coupled to conduction electron states near the Dirac point.
