Mossbauer and neutron diffraction studies on Co-Al ferrite

Abstract

Al substituted CoAlxFe1-xO4 (x = 0.1, 0.2, 0.3, and 0.5) have been studied with X-ray and neutron diffraction, Mossbauer 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 muB), Fe3+(B)(4.81 muB), Co2+(B)(2.99 muB), respectively. The temperature dependence of the magnetic hyperfine field in Fe-57 nuclei at the tetrahedral (A) and octahedral ( B) sites was analyzed based on the Neel theory of magnetism. In the sample CoAl0.1Fe1.9O4, the intersublattice A-B interaction and intrasublattice A-A superexchange interaction were antiferromagnetic with strengths of J(A-B) = -23.3 k(B) and J(A-A) = -17.6 k(B), respectively, while the intrasublattice B-B superexchange interaction was found to be ferromagnetic with a strength of J(B-B) = 5.5 k(B). 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.