Neutron Diffraction Study of Fe7Se8

Ferrimagnetic orderings of the moments of iron atoms in Fe 7 Se 8 are confirmed by neutron diffraction method; the moment is parallel to that of each atom on the same atomic layer parallel to the (001), while it is antriparallel to that on the next layer. When the temperature of the crystal is lowered from the Neel temperature (455°K), the direction of the moment, which is at first on the (001), changes towards the [001]. The behaviour of this “stand up” process depends on the type of the crystallographic superstructure of the specimen: gradually in the 4 c type crystal and abruptly in the 3 c type one. The ionic models of the magnetic structure, used in the case of pyrrhotite, for example, are not consistent with the result of the present experiment. A model with a more metallic character seems to be acceptable, which is supported by the Mossbauer effect experiment.