CuFe 2 O 4 has been studied by Mo È ssbauer spectroscopy and X-ray diffraction. The crystal is found to have a tetragonal spinel structure with the lattice constants a 0 (5.805 AE 0.005) # e and c 0 (8.669 AE 0.005) # e. The iron ions are in ferric Fe 3 states. The temperature dependence of the magnetic hyperfine fields at 57 Fe nuclei at the tetrahedral (A) and octahedral (B) sites is analyzed by the Ne  el theory of ferrimagnetism. The intersublattice superexchange interaction is found to be antiferromagnetic with its strength of J AÀB À19X5k B while intrasublattice superexchange interactions are ferromagnetic with their strengths of J AÀA 3X1k B and J BÀB 4X4k B .
(x=0.02, 0.05, 0.08) has been studied with Mössbauer spectroscopy and X-ray diffraction. The crystal structure is found to be a triclinic superstructure of the NiAs structure while Fe7 Se8 has a hexagonal structure. Abrupt change of quadrupole shifts near 124 K suggests that the spin-rotation transition proceeds abruptly, in contrast with the gradual transition reported for Fe7 Se8 with a triclinic superstructure. Isomer shifts indicate a ferrous character for the iron ions at all four sites. Both Néel and Debye temperatures are found to decrease with increasing cobalt concentration.
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