Charge disproportionation and antiferromagnetic order of Sr₃Fe₂O₇

The charge disproportionation and the magnetic properties of Sr₃Fe₂O₇, which has a double-layered perovskite structure, have been investigated. From ⁵⁷Fe Mössbauer measurements, the Néel temperature T_N and the critical temperature for the charge disproportionation reaction (2Fe⁴⁺ → Fe³⁺ + Fe⁵⁺) are determined to be 120 K ± 2 K and 343 K ± 10 K, respectively. Above 343 K, spectra clearly show a Fe⁴⁺ singlet. The ⁵⁷Fe Mössbauer spectrum at 4.5 K under externally applied longitudinal magnetic field consists of four magnetically split components and is similar to the spectrum obtained from CaFeO₃ with a helical spin structure. With an increase of the external magnetic field, the relative intensity of the 2nd and 5th absorption peaks increases, which implies that the magnetic moments of Fe have a tendency to form a texture perpendicular to the direction of the external magnetic field.