  We report on the crystal and magnetic structures, magnetic, and transport
properties of SrMnSb$_2$ single crystals grown by the self-flux method.
Magnetic susceptibility measurements reveal an antiferromagnetic (AFM)
transition at $T_{\rm N} = 295(3)$ K. Above $T_{\rm N}$, the susceptibility
slightly increases and forms a broad peak at $T \sim 420$ K, which is a typical
feature of two-dimensional magnetic systems. Neutron diffraction measurements
on single crystals confirm the previously reported C-type AFM structure below
$T_{\rm N}$. Both de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) effects
are observed in SrMnSb$_2$ single crystals. Analysis of the oscillatory
component by a Fourier transform shows that the prominent frequencies obtained
by the two different techniques are practically the same within error
regardless of sample size or saturated magnetic moment. Transmission electron
microscopy (TEM) reveals the existence of stacking faults in the crystals,
which result from a horizontal shift of Sb atomic layers suggesting possible
ordering of Sb vacancies in the crystals. Increase of temperature in
susceptibility measurements leads to the formation of a strong peak at $T \sim
{570}$ K that upon cooling under magnetic field the susceptibility shows a
ferromagnetic transition at $T_{\rm C} \sim 580$ K. Neutron powder diffraction
on crushed single-crystals does not support an FM phase above $T_{\rm N}$.
Furthermore, X-ray magnetic circular dichroism (XMCD) measurements of a single
crystal at the $L_{2,3}$ edge of Mn shows a signal due to induced canting of
AFM moments by the applied magnetic field. All evidence strongly suggests that
a chemical transformation at the surface of single crystals occurs above 500 K
concurrently producing a minute amount of ferromagnetic impurity phase.
