Auxiliary aqueous solution processes for the synthesis of cobalt ferrite with high coercivity at room temperature

Here, we describe the growth assisted (a) co-precipitation and (b) oxidation methods to achieve CoFe₂O₄ nanoparticles with high coercivity (H_c) and moderate magnetization (M_s) at room temperature (RT). The particle size is closely related to the relative interdependence between the nucleation and growth steps, which can be manipulated by the solution chemistry and precipitation conditions. Based on this premise, the particles synthesized by optimizing reaction parameters such as, reaction temperature, NaOH concentration and feed rate of metal ion supply source into the alkali solution, was used as seed to prepare particles as large as 40 nm in diameter. On the other hand, by optimizing the reaction parameters such as, concentrations of NaOH and oxidizing agent KNO₃, and the ratio of Fe³⁺/(Fe³⁺ + Fe²⁺), particles ranging from 15 to 110 nm were synthesized using the oxidation method. The Hc was enhanced from 0.56 kOe to 2.29 kOe at RT for particles synthesized via growth-assisted coprecipitation method using 1.13 mol of NaOH, metal ion feed rate of 0.06 mol/h at 98°C. Using the above single domain CoFe₂O₄ nanoparticles as seed during in-situ growth improved the Hc further to 2.9 kOe. On the other hand, among the particles synthesized with the growth assisted oxidation method, particles with diameter around 35 nm recorded the highest Hc of 2.02 kOe. The relation between Hc and particle diameter of CoFe₂O₄ synthesized using the above two methods was studied. The results suggested that size classified particles with 40 nm diameter obtained from the seeded particles showed the highest Hc of 4.3 kOe.