Mossbauer Studies of Cation Distribution in ZnxCo0.5-xFe2.5O4 Microspheres

The crystal structure of ZnxCo0.5-xFe2.5O4 (x = 0, 0.25, 0.5), prepared by a solvothermal reaction method, showed cubic spinel structure with space group Fd-3 m based on Rietveld refinement. The lattice constant a(0) increased linearly with the Zn concentration from x = 0 to 0.5. Field emission scanning electron microscope (FE-SEM) measurements showed that the size of the monodispersed particles was around 300-400 nm. With increasing Zn concentration, the saturation magnetization increased from 80.3 to 109.7 emu/g, while the coercivity at 293 K decreased from 893 to 46 Oe, respectively. The magnetocrystalline anisotropy constants (K-1) were determined as 1.62, 1.32, and 1.16 x 10(6) erg/cm(3) for x = 0, 0.25, and 0.5, respectively, based on the law of approach to saturations (LAS) method. We have investigated the cation distribution by Mossbauer spectroscopy, closely related to K-1. We have analyzed the recorded Mossbauer spectra as 3 sets with six-lines of tetrahedral A site, and octahedral B-1 and B-2 sites both at 4.2 and 293 K. From the isomer shift values, the valence states of A and B-1 site were determined to be ferric (Fe3+), while that at B-2 site to be ferrous (Fe2+). The corresponding area ratio of A site decreased from 40 to 30 % while that of site increased from 60 to 70% as the Zn concentration changed from x = 0 to 0.5 both at 4.2 and 293 K. Here, the changes in the area ratios of A and B sites are due to the changes in the cation distributions at the A and B sites, being originating from the randomly substituted Zn ions in ZnxCo0.5-xFe2.5O4 microspheres.