First Principles Calculation of Helical Spin Order in Iron Perovskite SrFeO3 and BaFeO3

Motivated by recent discovery of ferromagnetism in cubic perovskite BaFeO3 under small magnetic field, we investigate spin order in BaFeO3 and isostructual SrFeO3 by a first principles calculation using local spin density approximation (LSDA). We find that on-site Coulomb interaction U is necessary for obtaining helical spin order consistent with experiments. SrFeO3 exhibits G-type helical order, while BaFeO3 exhibits the A-type with very small wave vector. The A-type order is stabilized by lattice expansion. Small energy difference between the A-type and ferromagnetic orders explains ferromagnetism under small field. The LSDA+U results are consistent with model calculation where negative charge-transfer energy in these compounds is explicitly taken into account.