Moessbauer and Neutron Diffraction Studies on Co-Al Ferrite

摘要

Al substituted CoAl_xFe_(1-x)O_4 (x = 0.1, 0.2, 0.3, and 0.5) have been studied with X-ray and neutron diffraction, Moessbauer spectroscopy and magnetization measurements. Neutron diffraction at 10 K for CoAl_(0.1)Fe_(1.9)O_4 revealed a cubic spinel structure of ferrimagnetic long range ordering, with magnetic moments of Fe~(3+)(A)(-4.18 μ_B ), Fe~(3+)(B)(4.81 μ_B), Co~(2+)(B)(2.99 μ_b), respectively. The temperature dependence of the magnetic hyperfine field in ~(57)Fe nuclei at the tetrahedral (A) and octahedral (B) sites was analyzed based on the Neel theory of magnetism. In the sample CoAL_(0..1)-Fe_(1.9)o_4, 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 Fe~(3+)(A) and a noticeable strength of the A-A interaction are closely related to the covalency effects.