Effects of Co-substitution on the structural and magnetic properties of NiCoxFe2−xO4 ferrite nanoparticles

The sol–gel auto-combustion method was used to prepare nanocrystalline powders of Co-substituted nickel ferrite with the general formula NiCoxFe2−xO4 (x=0.0, 0.1, 0.25, and 0.5). The effects of Co-doping on the structural, morphological, and magnetic properties of the samples were subsequently evaluated by X-ray diffraction (XRD), Fourier transform infrared (FTIR), field emission scanning electron microscopy (FE-SEM), and vibrating sample magnetometer (VSM). Using the MAUD program, the full pattern fitting of Rietveld method was employed to determine the exact coordinate of the atoms, unit cell dimensions, and ion occupancy. X-ray diffraction measurements by Rietveld refinement confirmed the crystalline structure and phase purity of all the ferrites prepared. FTIR results also confirmed the formation of a spinel phase and FE-SEM images showed that the particles were in the nanosize range. Moreover, Rietveld analysis and saturation magnetization (Ms) revealed that Co3+ replaced Fe3+ in the tetrahedral A-sites up to x=0.1. then, it replaced Fe3+ in both A- and B-sites for x≥0.25. Finally, VSM results demonstrated that while Ms remained nearly constant with increasing Co3+ substitution, coercivity (Hc) increased significantly. It may be suggested that the larger magneto-crystalline anisotropy of Co3+ ions is responsible for the increased Hc observed in the Co-doped Ni ferrite samples.