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.