The role of shell thickness on the exchange spring mechanism of cobalt ferrite/iron cobalt magnetic nanocomposites

CoFe2O4/FeCo hard/soft core-shell system with different shell thickness was obtained by precise control of surface treatment conditions of the CoFe2O4 nanoparticles under reduced condition. CoFe2O4 nanoparticles were subjected to surface reduction treatment for various period of times (10, 15, 20, 25, and 30 min) at 550 °C in order to optimize the thickness of FeCo soft shell. The phase evolution, morphology, coercivity and magnetization of the nanocomposites were examined by X-ray diffraction, electron diffraction pattern, transmission electron microscope, and vibrating sample magnetometer. The single-phase-like hysteresis loops and Henkel plot reflect the existence of the exchange coupling phenomenon in the core-shell nanocomposite. The enhancement of maximum energy product, (BH)max, by 50% with respect to the pure CoFe2O4 nanoparticles is obtained for a sample treated at 20 min.