Abstract
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.
