Al substituted CoA1 x Fel−x O4 (x = 0.1, 0.2, 0.3, and 0.5) have been studied with X-ray and neutron diffraction, Mössbauer spectroscopy and magnetization measurements. Neutron diffraction at 10 K for CoAl0.1Fe1.9O4 revealed a cubic spinel structure of ferrimagnetic long range ordering, with magnetic moments of Fe3+(A)(−4.18 μ B), Fe3+(B)(4.81 μ B), Co2+(B)(2.99 μ B), respectively.
The temperature dependence of the magnetic hyperfine field in 57Fe nuclei at the tetrahedral (A) and octahedral (B) sites was analyzed based on the Néel theory of magnetism. In the sample CoA10.1-Fel1.9O4, the intersublattice A-B interaction and intrasublattice A-A superexchange interaction were antiferromagnetic with strengths of J A-B = −2.3 kB and J A-A = −17.6 kB, respectively, while the intrasublattice B-B superexchange interaction was found to be ferromagnetic with a strength of J B-B = 5.5 kg. With increasing Al substitution the A-B and B-B interaction decreased but the A-A interaction increased. It is interpreted that the reduction of magnetic moment in Fe3+(A) and a noticeable strength of the A-A interaction are closely related to the covalency effects.