Growth dominant co-precipitation process to achieve high coercivity at room temperature in CoFe₂O₄ nanoparticles

Applications of CoFe₂O₄ are limited due to the lack of synthesis technique to produce monodispersed, single domain and high coercivity (H_c) nanoparticles. Here, we describe the growth dominant co-precipitation process to achieve high H_c with moderate magnetization at room temperature (RT) in CoFe₂O₄ nanoparticles. It is well known that the particle size is closely related to the relative interdependence between the nucleation and growth steps, which in turn can strongly be affected by the solution chemistry and precipitation conditions. Based on this premise, the effect of 1) reaction temperature, 2) NaOH concentration, and 3) feeding rate of metal ions into the alkali solution were evaluated. The maximum H_c of 2.29 kOe (RT) was observed for the CoFe₂O₄ prepared at 98 °C, 1.13-mol NaOH, and the metal ion feeding rate of 0.00103 M/min. To improve the coercivity, single domain CoFe₂O₄ nanoparticles were produced by in situ growth of the CoFe₂O₄ seeds followed by size separation method. A coercivity of 4.3 kOe was achieved at RT for 40-nm single domain CoFe₂O₄ nanoparticles, which is close to the theoretical value of 5.3 kOe.