Charge disproportionation and antiferromagnetic order of Sr3Fe2O7

The charge disproportionation and the magnetic properties of Sr3Fe2O7, whic h has a double-layered perovskite structure, have been investigated. From F e-57 Mossbauer measurements, the Neel temperature TN and the critical tempe rature for the charge disproportionation reaction (2Fe(4+) --> 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 Fe-57 Mossbauer spectrum a t 4.5 K: under externally applied longitudinal magnetic field consists of f our magnetically split components and is similar to the spectrum obtained f rom 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 fie ld.