Synthesis and Magnetic Properties of Magnetic Fluids Based on Co\({}_{\boldsymbol{x}}\)Ni\({}_{\boldsymbol{1-x}}\)Fe\({}_{\boldsymbol{2}}\)O\({}_{\boldsymbol{4}}\) Nanoparticles

Magnetic nanoparticles Co\({}_{x}\)Ni\({}_{1-x}\)Fe\({}_{2}\)O\({}_{4}\) with \(x=0\), \(0.25\), \(0.5\), \(0.75\) were synthesized by using the chemical coprecipitation method and magnetic fluids based on them were prepared. A structural and phase analysis of the obtained nanoparticles was performed, the characteristic sizes of nanoparticles and size dispersion, the magnetization of magnetic fluids containing a 0.5\(\%\) volume concentration of magnetic nanoparticles were determined. Diffraction patterns of all compositions confirm the formation of a single-phase structure of cubic spinel NiFe\({}_{2}\)O\({}_{4}\) and CoFe\({}_{2}\)O\({}_{4}\). The average particle diameters determined using a transmission electron microscope are 17.22, 15.04, 14.69, 11.18 nm at \(x=0\), \(0.25\), \(0.5\), \(0.75\), respectively. The saturation magnetization of magnetic fluids based on Co\({}_{x}\)Ni\({}_{1-x}\)Fe\({}_{2}\)O\({}_{4}\) nanoparticles decreases from 38.25 to 5.33 emu/kg as the cobalt concentration decreases from \(x=0.75\) to \(x=0\). For \(x=0\), \(0.25\), and \(0.5\), magnetic fluids are characterized by superparamagnetic properties, whereas for Co\({}_{0.75}\)Ni\({}_{0.25}\)Fe\({}_{2}\)O\({}_{4}\)-based fluids, a wide hysteresis loop with a coercive force H\({}_{C}=612\) Oe was observed, which is associated with the single-domain state of Co\({}_{0.75}\)Ni\({}_{0.25}\)Fe\({}_{2}\)O\({}_{4}\) particles despite their small size.