Re-creation of single phase, and improvement of magnetic property of CoFe2O4 nanoparticles versus heat treatment

Our studies on the crystal characterization and magnetic property of CoFe₂O₄ nanoparticles (NPs) point out their instability in a specific temperature range. While as-prepared NPs exhibit single phase in a cubic spinel structure, annealing at temperatures T=673–1273 K leads to the development of an impurity phase of Fe₂O₃. Interestingly, annealing at higher temperatures re-creates the single phase of NPs. This strongly influences their magnetic property. The magnetic inhomogeneity and/or multiple phase exist in as-prepared NPs and in those annealed below 1273 K, better magnetic property is found in the samples with annealing temperature (T_an) higher than 1273 K. Ferromagnetic-paramagnetic phase transition temperatures of these samples are located around 815–850 K, and are less dependent on T_an. At room temperature, their saturation magnetization is located in the range of 41–55 emu/g, while the coercivity can be changed from 600 to ~3200 Oe. These results are related to microstructures, structural phases, and exchange interactions between Fe and Co ions situated in the A and B sites of the spinel structure, which are modified by heat treatment.