Mossbauer studies of superexchange interactions and atomic migration in CoFe2O4

The Co ferrite, CoFe2O4, has been investigated by X-ray diffraction and Mos sbauer spectroscopy. The crystal structure is found to be an inverse cubic spinel with the lattice constants a(0) = 8.381 +/- 0.005 Angstrom and 8.391 +/- 0.005 Angstrom for slowly cooled and quenched CoFe2O4 respectively. Th e iron ions are in ferric Fe3+ states. The temperature dependence of the ma gnetic hyperfine fields of Fe-57 at the tetrahedral (A) and octahedral (B) sites is analyzed by the Neel theory of ferrimagnetism. For the slowly cool ed 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 ferr omagnetic 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 fro m A to B sites. (C) 2000 Elsevier Science B.V. All rights reserved.