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 ≈ 300 K in k = 0 antiferromagnetic structures, with ordered Mn moments at T = 10 K of approximately 3.8 µB aligned along the c axis. The magnetic structures are Néel-type within the Mn-Bi layers but the inter-layer ordering is different, being antiferromagnetic in SrMnBi2 and ferromagnetic in CaMnBi2. This allows a meanfield 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.

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