  Eu-based Zintl-phase materials EuA$_2$Pn$_2$ (A = Mg, In, Cd, Zn; Pn = Bi,
Sb, As, P) have generated significant recent interest owing to the complex
interplay of magnetism and band topology. Here, we investigated the electronic,
magnetic, and electronic properties of the layered Zintl-phase single crystals
of EuMg$_2$Sb$_2$ with the trigonal CaAl$_2$Si$_2$ crystal structure (space
group $P\bar{3}m1$). Electrical resistivity measurements complemented with
angle-resolved photoemission spectroscopy (ARPES) studies find an activated
behavior with the intrinsic conductivity at high temperatures indicating a
semiconducting electronic ground state with a narrow energy gap of 370 meV.
Magnetic susceptibility and zero-field heat-capacity measurements indicate that
the compound undergoes antiferromagnetic (AFM) ordering at the Neel temperature
$T_{\rm N}$ = 8.0(2) K. Zero-field neutron-diffraction measurements reveal that
the AFM ordering is A-type where the Eu ordered moments (Eu$^{2+}$, S= 7/2)
arranged in ab-plane layers are aligned ferromagnetically in the ab plane with
the Eu moments in adjacent layers aligned antiferromagnetically. We also find
that Eu-moment reorientation in the trigonal AFM domains within the ab planes
occurs below $T_{\rm N}$ at low fields < 0.05 T due to very small in-plane
anisotropy. Although isostructural semimetallic EuMg$_2$Bi$_2$ is reported to
host Dirac surface states, the observation of narrow-gap semiconducting
behavior in EuMg$_2$Sb$_2$ implies a strong role of spin-orbit coupling in
tuning the electronic states of these materials.
