Cation Distributions in Magnesium-Nickel Ferrites

Abstract

The dependence of the nonthermal part of the internal energy (u1) of mixed magnesium-nickel ferrites, MgyNi1−yFe2O4 on the cation distribution has been studied. The cation distribution,which is charactered by the parameters η (the fraction of Mgions on tetrahedral sites) and ξ (the fraction of Ni ions on tetrahedral sites), is determined from measurements of the saturation magnetization of samples quenched from temperatures between 1400° and 400°C. As in the case of magnesium-manganese ferrites it is found that u1 = u0+R(θ0−12θ1η2). The parameters θ0 and θ1 are related to the energies necessary to transport Mg ions from octahedral (B) to tetrahedral (A) sites. The Madelung energy portion of θ0 predicts a linear dependence of θ0on y and this is observed experimentally. It is also demonstrated that θ0 is increased considerably by the substitution of Ni ions for Mn ions in the spinel lattice as dictated by the Madelung energy. The dependence of θ1 on y can also be qualitatively explained. It is shown that the magnetic moment data considered as a function of composition can give useful information about the degree of inversion in the spinel lattice despite the two degrees of inversional freedom (ξ and η). The variation of the maximum attainable value of η with composition depends on ξ and the magnetic moment of the Ni ion. Values of ξ and the Ni ionic magnetic moment compatible with the observed data are discussed.