Effect of Synthesis Method on Particle Size and Magnetic and Structural Properties of Co-Ni Ferrites

Mössbauer spectroscopy, as well as magnetic measurements and transmission electron microscopy, was employed to investigate the particle size and magnetic and structural properties of Co-Ni spinel ferrites synthesized via a succession of chemical co-precipitation, hydrothermal treatment, and etching in hydrochloric acid (HCl). A maximum coercive force of 519 kA/m and saturation magnetization of 60.4~\text {A}\cdot \text {m}^{2} /kg were obtained after etching in HCl with a concentration of 4 mol/L. At room temperature, the samples obtained after co-precipitation showed partially paramagnetic behavior; however, this effect disappeared in the Mössbauer spectra at liquid He temperature. The area ratio of the Fe ions at A-sites and B-sites changed as particle size was increased, indicating that the cation distribution is changed as a result of the size and synthesis method. After the final stage of synthesis, i.e., etching, the degree of inversion was found to be 1, indicating that the structure is a completely inverse spinel. It is expected that the cation distribution, especially the increase of Co 2+ in the octahedral site, produces a higher magnetocrystalline anisotropy and thus higher coercivity.