Particle interactions in SrFe12O19/NiFe2O4 magnetic composites: Effect of composition ratios and nickel ferrite size

The magnetic interactions in SrFe₁₂O₁₉/NiFe₂O₄ composites have been studied using the component fractions (90/10, 85/15, 80/20, 75/25 wt%) and the size of magnetically soft nickel ferrite particles as variables. It was observed that the greater the amount of nickel ferrite (NFO) in the composites, the stronger the intensity of the dipolar interactions. Reducing the NFO particle size (from 190 nm to about 25 nm) resulted in an improvement in the M_s and M_r values of the composite, despite the fact that the M_s of NFO is smaller than the M_s of the hexaferrite ancestor (SrM). This fact is attributed to the diffusion of Ni atoms into the SrM lattice as a result of the heat treatment following the mechanosynthesis of the two ferrites, the method of their fine mixing. An increase in the net magnetization of the composite can be explained by the substitution of Fe³⁺ ions by Ni²⁺ ions in the 4f₁ and/or 4f₂ positions of the SrM lattice. Such an exclusive exchange takes place in the complex system only when small nickel ferrite particles are present in the composite. That is, their reactivity is increased compared to larger ones.