We use resonant elastic x-ray scattering to determine the evolution of magnetic order in below K, as a function of temperature and applied magnetic field. We find an A-type antiferromagnetic structure with in-plane magnetic moments, and observe dramatic magnetoresistive effects associated with field-induced changes in the magnetic structure and domain populations. Our ab initio electronic structure calculations indicate that the Dirac dispersion found in the nonmagnetic Dirac semimetal is also present in , but is gapped for due to the breaking of symmetry by the magnetic structure.