Exchange-spring effect in CoFe2O4/CoFe2 composite nano-particles

• CoFe2O4/CoFe2 composites were synthesized by reducing CoFe2O4 at 300 °C and 400 °C.
• Heating temperature makes a great impact on composition and magnetic properties.
• Inter-particle dipole interaction is dominant in the product of heating at 300 °C.
• Inter-particle exchange interaction appears in the product of heating at 400 °C.

CoFe2O4/CoFe2 composite nano-particles were synthesized by heating CoFe2O4 nano-particles in reducing atmosphere at 300 °C and 400 °C. Their composition, microstructure and magnetic properties were investigated. It was found that the product of reducing CoFe2O4 at 300 °C is composed by CoFe2O4 nano-particles and a small amount of CoFe2 nano-alloy. Based on the negative values of Henkel plots and the two-phase behavior in magnetic hysteresis loop the dominant inter-particle dipole coupling was confirmed. But in the process of reducing CoFe2O4 at 400 °C, surface layers of CoFe2 were formed on the CoFe2O4 nano-particles, and exchange coupling appeared at the interface of both phases, which was confirmed from the positive values of Henkel plots.