Magnetic properties of core/shell-structured CoFe2/CoFe2O4 composite nano-powders synthesized via oxidation reaction

The CoFe2/CoFe2O4 composite is a new type of exchange-spring magnet that has attracted much attention in recent years. To resolve the unstable chemical properties for CoFe2/CoFe2O4 composite nanoparticles with CoFe2 shells prepared by reducing CoFe2O4, we synthesized core/shell-structured CoFe2/CoFe2O4 composite nanoparticles with CoFe2O4 shells by oxidizing CoFe2 nanoparticles. The composition and micro-structure of the composite were characterized using an X-ray diffractometer, a transmission electron microscope and a high-resolution transmission electron microscope. Magnetic measurements, including magnetic hysteresis loops, Henkel plots, and temperature-dependent magnetization curves, were performed using a vibrating sample magnetometer. The results showed that the nanoparticles were composed of CoFe2 cores and CoFe2O4 shells, and a regular non-monotonous variation of properties was found. When the mass percentage of CoFe2O4 was 30%, the average crystallite size of CoFe2 reached its maximum value, a minimum coercivity was observed, and the exchange coupling between CoFe2 and CoFe2O4 was strengthened. This non-monotonous change in properties is attributed to the lattice vacancies and spin-glass-like state occurring when the mass percentage of CoFe2O4 is very small and dominant hard magnetic behavior exists in the larger CoFe2O4 phase.