Effects of mass ratio of CoFe₂ to CoFe₂O₄ and the dilution by MgO on the magnetic properties of CoFe₂@CoFe₂O₄

MgO coated CoFe₂O₄ nanoparticles (CoFe₂O₄@MgO) were synthesized by the thermal decomposition of a metal-organic salt. CoFe₂O₄@MgO was reduced in H₂ and then oxidized in air to prepare a group of CoFe₂@CoFe₂O₄@MgO samples. Then MgO was dissolved by the HCl solution to obtain the other group of samples (CoFe₂@CoF₂O₄). The magnetic field and temperature dependent magnetization curves were measured on CoFe₂@CoF₂O₄ and CoFe₂@CoFe₂O₄@MgO samples. The significant changes of coercivity H_c, saturation magnetization M_s and remanence ratio M_r/M_s as well as the magnetic ordering states with the oxidation temperature are observed for CoFe₂@CoF₂O₄ and CoFe₂@CoFe₂O₄@MgO samples, which strongly depend on the anisotropy of the magnetic particles and interparticle dipolar interaction.