  We investigate the magnetic order in single crystalline EuCd$_2$Sb$_2$ thin
films using a combined theoretical and experimental approach. Resonant elastic
x-ray scattering experiments reveal a sharp magnetic peak at $q = (0, 0,
\frac{1}{2})$ below $T_N = 7.2$ K, indicative of interlayer antiferromagnetic
ordering. Additionally, we observe a weak diffuse magnetic signal centered at
$q = (0, 0, 1)$ that persists above $T_N$, up to $T_C \sim 11$ K. Our
Monte-Carlo simulations of a classical spin model approximation of the Eu
magnetic sublattice demonstrate that the diffuse signal can arise from
ferromagnetic coupling in the top few layers due to surface oxidation. On the
other hand, the bulk of the sample exhibits antiferromagnetic coupling between
layers. Finally, our fit of the model parameters to the magnetic ordering
temperatures, shed light on the exchange couplings that are key in stabilizing
the observed composite magnetic order.
