  Crystals of SrMn2Sb2 and BaMn2Sb2 were grown using Sn flux and characterized
by powder and single-crystal x-ray diffraction, respectively, and by
single-crystal electrical resistivity rho, heat capacity Cp, and magnetic
susceptibility chi measurements versus temperature T, and magnetization versus
field M(H) isotherm measurements. SrMn2Sb2 adopts the trigonal CaAl2Si2-type
structure whereas BaMn2Sb2 crystallizes in the tetragonal ThCr2Si2-type
structure. The rho(T) data indicate semiconducting behaviors for both compounds
with activation energies of 0.35 eV for SrMn2Sb2 and 0.16 eV for BaMn2Sb2. The
chi(T) and Cp(T) data reveal antiferromagnetic (AFM) ordering at TN = 110 K for
SrMn2Sb2 and 450~K for BaMn2Sb2. The anisotropic chi(T < TN) data also show
that the ordered moments in SrMn2Sb2 are aligned in the hexagonal ab plane
whereas the ordered moments in BaMn2Sb2 are aligned collinearly along the
tetragonal c axis. The ab-plane M(H) data for SrMn2Sb2 exhibit a continuous
metamagnetic transition at low fields 0 < H < 1 T, whereas BaMn2Sb2 exhibits no
metamagnetic transitions up to 5.5 T. The chi(T) data for both compounds and
the Cp(T) data for SrMn2Sb2 and BaMn2Sb2 indicate strong dynamic short-range
AFM correlations above their respective TN up to at least 900 K within a
local-moment picture, corresponding to quasi-two-dimensional magnetic behavior.
The present results and a survey of the literature for Mn pnictides with the
CaAl2Si2 and ThCr2Si2 crystal structures show that the TN values for the
CaAl2Si2-type compounds are much smaller than those for the ThCr2Si2-type
materials.
