  We reveal by first-principles calculations that in iron pnictides
Sr4M2O6CrFeAs2 (M=Sc, Cr) the two-dimensional CrFeAs2 layers exhibit a robust
band structure of half-metallic antiferromagnet (HMAFM). Due to the thick
blocking layer, the interlayer coupling is vanishingly small and thus the
conductive channels in individual layers may take the alternative spin
direction randomly. We show that, since the spin magnetizations of Fe and Cr
are different in a ferromagnetic state, applying a strong magnetic field and
ramping it down gradually can align the spin direction of conductive channel of
all HMAFM layers, which restores the bulk HMAFM.
