Effect of Mg-substitution in Co-Ni-Ferrites: Cation distribution and magnetic properties

We report the changes in structural and magnetic properties of spinel ferrites by doping magnesium, a cation that occupies the tetrahedral site, in place of nickel that occupies the octahedral site in Co-Ni ferrite (Co0.5Ni0.5Fe2O4) ceramic. We synthesized nano-crystalline Co0.5MgxNi0.5-xFe2O4 (x = 0, 0.1, 0.2, 0.3, 0.4) ceramic powder samples by sol-gel autocombustion method followed by calcination at 600 degrees C in air for 2 h. X-ray diffraction (XRD) patterns of the synthesized samples confirm crystalline single-phase spinel structure with cubic symmetry (Fd-3m space group). Refined structural parameters were calculated through Rietveld refinement of the XRD patterns. The crystallite sizes for all the samples were found to be in the range of 30-38 nm. Porous morphology of the samples was clearly observed from the scanning electron micrographs. The FTIR spectra confirm the formation of the spinel phase through the observed vibrational bands assigned to the tetrahedral (T-d) and octahedral (O-h) interstitial complexes in the spinel structure. Magnetic measurements indicate a decrease of saturation magnetization (M-s) with increase in concentration of Mg. Room temperature Fe-57-Mossbauer spectroscopy in association with XRD results confirmed the ferrimagnetic nature of the samples with Mg occupying the T-d site, Ni occupying the O-h site and Co occupying both the T-d and O-h sites in the ratio of 2:3. Using the cation occupancies, Neel's two sublattice model could explain the observed magnetic properties in our Mg doped Co-Ni ferrite samples.