Preparation of exchange coupled CoFe2O4/CoFe2 nanopowders

•The CoFe2O4 magnetic nanoparticles were synthesized by hydrothermal method.•The CoFe2 nanoparticles were prepared by reducing CoFe2O4 in H2 flow.•The exchange coupling of 90%wt. CoFe2O4/10%wt. CoFe2 was observed clearly in the optimal nanocomposite powders.•The highest energy product of the prepared 90%wt. CoFe2O4/10%wt. CoFe2 nanocomposite powders reached only 15.8 kJ/m3. The CoFe2O4 cobalt ferrite nanoparticles were synthesized by the hydrothermal method. At the reaction temperature of 200 °C, the particle sizes were controlled in the range of 22–41 nm by varying the reaction time tr from 2.0 to 15 h and the spontaneous magnetization Ms of samples was increased from 68.6 to 78.2 Am2/kg. The X-ray diffraction diagrams of obtained samples showed their pure ferrite spinel structure. The highest coercivity Hc value of 184.5 kA/m was achieved by tr = 8 h (denoted by CFO). The sample was synthesized with a time reaction of 2 h which was reduced to form CoFe2 (denoted by CF) nanoparticles by H2 flow 300 mL/min at 380 °C. The CFO/CF powders were milled by the high energy mechanical ball milling in a SPEX 8000. After milling for 2 h, the Ms and Hc values of the CFO/CF nanocomposite powders with 10 wt% CoFe2 nanoparticles fraction were 81.5 Am2/kg and 192.4 kA/m, respectively. The microstructure and magnetic properties of all samples are discussed in detail.