Mössbauer studies of superexchange interactions and atomic migration in CoFe2O4

The Co ferrite, CoFe 2 O 4 , has been investigated by X-ray diffraction and Mossbauer spectroscopy. The crystal structure is found to be an inverse cubic spinel with the lattice constants a 0 = 8.381 ± 0.005 A and 8.391 ± 0.005 A for slowly cooled and quenched CoFe 2 O 4 , respectively. The iron ions are in ferric Fe 3+ states. The temperature dependence of the magnetic hyperfine fields of 57 Fe at the tetrahedral (A) and octahedral (B) sites is analyzed by the Neel theory of ferrimagnetism For the slowly cooled sample, the A-B intersublattice superexchange interaction is found to be antiferromagnetic with a strength of J A-B = -24.4k B . while A-A and B-B intrasublattice superexchange interactions are antiferromagnetic and ferromagnetic with J A-A = - 18.2k B and J B-B = 3.9k B , respectively. For the quenched sample J A-B = -23.6k B J A-A = - 17.8k B . and J B-B = 3.9k B are found. The decrease of the Mossbauer absorption area ratio of A to B patterns above 400 K is explained in terms of migrating iron ions from A to B sites.