Abstract
Nakahara et al. [Mater. Lett. 30 (1997) 163–167] have refined their X-ray diffraction patterns of Ca 2 FeMnO 5 with the brownmillerite structure, in which all the octahedral in-plane sites are occupied by Mn, and all the tetrahedral in-chain sites are occupied by Fe. Our Mössbauer study of Ca 2 FeMnO 5 is in agreement with this cationic distribution. We determine the spin orientation with respect to the crystal axes and suggest the model of layered magnetic structure. From our Mössbauer spectra and from analysis of the crystal and magnetic structures in a number of brownmillerite-related oxides we suggest for Ca 2 FeMnO 5 the mixed-spin S = 5 2 and S = 2 Ising system. The Fe 3 + and Mn 3 + moments order antiferromagnetically (AFM) both in-plane and in perpendicular direction and aligned up and down vertically in the out-of-plane direction. Although we cannot exclude a small tilt and a related small ferromagnetic component, basically, each of Fe and Mn spin subsystems generate individually a C-type sublattice and two these sublattices are superposed to give an unusual G-like type structure with two sorts of spins. Nuclear forward scattering (NFS) and nuclear inelasting scattering (NIS) techniques were used to characterize, respectively, the hyperfine interactions and phonon density of states (DOS) of 57Fe. Comparing the vibrational properties of several brownmillerites we were able to give an approximate DOS for tetrahedral and octahedral positions.
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