  We report powder and single crystal neutron diffraction measurements of the
magnetic order in AMnBi2 (A = Sr and Ca), two layered manganese pnictides with
anisotropic Dirac fermions on a Bi square net. Both materials are found to
order at TN approx 300 K in k = 0 antiferromagnetic structures, with ordered Mn
moments at T = 10 K of approximately 3.8 muB aligned along the c axis. The
magnetic structures are Neel-type within the Mn--Bi layers but the inter-layer
ordering is different, being antiferromagnetic in SrMnBi2 and ferromagnetic in
CaMnBi2. This allows a mean-field coupling of the magnetic order to Bi
electrons in CaMnBi2 but not in SrMnBi2. We find clear evidence that magnetic
order influences electrical transport. First principles calculations explain
the experimental observations and suggest that the mechanism for different
inter-layer ordering in the two compounds is the competition between the
anteiferromagnetic superexchange and ferromagnetic double exchange carried by
itinerant Bi electrons.
