Pressure-induced ferromagnetism in cubic perovskite SrFeO3 and BaFeO3

The spin order in cubic perovskite SrFeO3 and BaFeO3 under high pressure is studied by density functional theory (DFT) calculation with local spin density approximation plus Hubbard U (LSDA + U). At ambient pressure, A-type and G-type helical spin orders are almost degenerate in BaFeO3 whose lattice constant is 3.97 angstrom. When the lattice constant is reduced to 3.85 angstrom, which is the same as the lattice constant of SrFeO3 at ambient pressure, G-type helical spin order becomes stable, being consistent with SrFeO3. This is because superexchange interaction is enhanced as compared with double-exchange interaction. Phase transition from helical spin state to ferromagnetic state in both SrFeO3 and BaFeO3 takes place if the lattice constant is further reduced to 3.70 angstrom. This is because reduced local spin moment weakens the contribution from superexchange interaction. Our result agrees with recent experimental result of BaFeO3 under high pressure. Additionally, our calculation predicts that half-metal BaFeO3 at ambient pressure will become a good metal under high pressure.