Magnetic phase diagram of Sr3Fe2O7-δ

Magnetometry, electrical transport, and neutron scattering measurements were performed on single crystals of the Fe4+-containing perovskite-related phase Sr3Fe2O7-delta as a function of oxygen content. Although both the crystal structure and electron configuration of this compound are closely similar to those of well-studied ruthenates and manganates, it exhibits very different physical properties. The fully oxygenated compound (delta = 0) exhibits a charge-disproportionation transition at T-D = 340 K, and an antiferromagnetic transition at T-N = 115 K. For temperatures T <= T-D, the material is a small-gap insulator; the antiferromagnetic order is incommensurate, which implies competing exchange interactions between the Fe4+ moments. The fully deoxygenated compound (delta = 1) is highly insulating, and its Fe3+ moments exhibit commensurate antiferromagnetic order below T-N similar to 600 K. Compounds with intermediate delta exhibit different order with lower T-N, likely as a consequence of frustrated exchange interactions between Fe3+ and Fe4+ sublattices. A previous proposal that the magnetic transition temperature reaches zero is not supported.