The influence of cation distribution on the magnetic properties of mixed Co1-yNiyFe2O4 nanoferrites produced by the sol-gel method

Highlights:• Cobalt nickel ferrite (Co_yNi_1-yFe₂O₄) nanoparticles were produced via sol-gel synthesis.• Nickel-rich samples form an inverted spinel structure.• Coercivity increases with anisotropy energy and decreases with inversion degree.• A 25% replacement of Ni²⁺ with Co²⁺ enhances coercivity over six times.• The increase in Co⁺² content generated a net increase in the magnetic moments. -- Abstract: Mixed nanoferrites have been increasingly used as functional materials due to their versatile magnetic properties, associated with high surface energies. This work studied a series of mixed (Co_1-yNi_y)Fe₂O₄ nanoferrites with y = 0, 0.25, 0.5, 0.75 and 1.0, which were aimed to finely tune their magnetic properties by using composition manipulation. The synthesis was conducted via a nitrate/citrate sol-gel method associated with low annealing temperature, which provided easy control of cation distribution in the nanostructure. XRD results confirmed the formation of a cubic spinel phase for all the samples. TEM and BET results further confirmed the nanoscale (11–16 nm) size of the obtained particles of the materials. Raman and Mössbauer techniques allowed for the determination of the spinel inversion degree of the samples by estimating the population of Fe³⁺ cations at octahedral (B) and tetrahedral (A) sites. Nickel ferrite is arranged in an inverse spinel framework with soft magnetic behavior (H_c=83 Oe; M_s=41.72 emu.g⁻¹), while the cobalt ferrite sample (CoFe₂O₄) shows a hard magnetic behavior (H_c=894 Oe; M_s=51.13 emu.g⁻¹) with a structure of a partially inverse spinel. The considerable magnetic hardening upon Co²⁺ substitution is explained considering the increase in anisotropy energy and the decrease in inversion degree, which generate a net growth in the magnetic moment. This study is a further step towards understanding the inversion mechanisms involved in the fine-tuning of ferrimagnetic compounds.