In this work, a new two-stage scalable method for the synthesis of magnetite nanoparticles for biomedical applications is proposed. The influence of the milling time, medium, and surfactants on the formation of the structure, magnetic, and functional properties of magnetite nanoparticles has been studied. Comprehensive investigation of the formation of the structure and properties of magnetite nanoparticles has been carried out using X-ray diffraction analysis, scanning and transmission electron microscopy, Mössbauer spectroscopy, measurements of magnetic properties, specific loss power (SLP), and cytotoxicity. It was shown that the milling medium of water with the addition of trisodium citrate is a harsher milling condition compared to octadecene-1 with the addition of oleic acid. Continuous milling for 50 h allowed to obtain a fraction of colloidally stable nanoparticles at the level of 80–90%. Harsher milling conditions led to the formation of a larger fraction of superparamagnetic particles, which reduced the coercivity and SLP. The maximum SLP value of 1140 W/g was reached by large particles, while nanoparticles had decreased SLP values of 100–190 W/g, which was completely determined by the coercivity dependence. Different synthesis conditions allowed obtaining particles with different cytotoxicity against PC-3 cells.

At present, strontium hexaferrite is one of the promising materials for production of permanent magnets [1–3], electrical and microwave devices [4–5], etc. In this regard, the study of Ca-substituted strontium hexaferrite is of particular interest since calcium is cheaper than strontium and will allow reducing the production cost of the material. However, it is necessary to study the effect of such doping on the hexaferrite structure and properties. Hexaferrite also has prospects as a material for biomedical applications. However, when hexaferrite decomposes in the human body, strontium can partially replace calcium in bones, while excess of strontium and a lack of calcium can, in turn, lead to negative consequences for the human body [6–15]. Therefore, specimens of Ca-substituted strontium hexaferrite have been studied in this work. Specimens composed of Sr\(_{{1 - x}}\)Ca_xFe₁₂O₁₉ (x = 0, 0.1, 0.2, 0.3, 0.5, 0.75, and 1) have been obtained by the mechanochemical method, followed by annealing at 1200°C. The specimens have been studied employing phase and X-ray diffraction analyses. The curves reflecting change in the lattice periods and the volume of the unit cell V of strontium hexaferrite have been constructed depending on the degree of calcium doping. The miscibility gap of Ca in strontium hexaferrite at 1200°C has been determined. The magnetic properties of the obtained specimens have been investigated. It has been found that calcium doping does not cause a decrease in saturation magnetization. It is concluded that calcium doping can be used to reduce the cost of manufacturing a material and increase its biocompatibility without the risk of reducing its functional properties because of reduced magnetization.