Effect of annealing temperature on magnetic phase transition in Fe3O4 nanoparticles

• Fe3O4 nanoparticles (NPs) were synthesized using a co-precipitation method.
• Prepared NPs annealed at 50, 250, 350, 450, 550, 650, 750, and 850°C for 1h in air.
• Transform of superpara Fe3O4 to superpara γ-Fe2O3 occurred by annealing at 250 °C.
• Superpara Fe3O4 transformed into α-Fe2O3, with soft ferromagnetic phase at 650 °C.
• Superparamagnetic Fe3O4 was transformed into hard ferromagnetic α-Fe2O3 at 850 °C.

Fe3O4 (magnetite) nanoparticles (NPs) were synthesized using a co-precipitation method, and then annealed at various temperatures between 50 and 850 °C for 1 h in air. After annealing, the NPs were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and vibrating sample magnetometer (VSM). The FTIR and XRD results indicated that Fe3O4 NPs were converted to γ-Fe2O3 (maghemite) by annealing at 250 °C for 1 h and then to α-Fe2O3 (hematite) on annealing in the range of 550–650 °C. The average crystallite size of the NPs estimated by the Debye–Scherrer equation increased from 6.6 to 37.6 nm by increasing annealing temperature from 50 to 850 °C. According to VSM results, the magnetite NPs were superparamagnetic and converted to the maghemite with superparamagnetic phase by annealing up to 550 °C. A phase transition from soft to hard ferromagnetic was occurred at annealing temperature 650 and 850 °C, respectively °C. This phase transition was attributed to the conversion of magnetite to hematite. The VSM analysis confirmed the XRD and FTIR results. The saturation magnetization (Ms) of Fe3O4 NPs was increased from 41.69 to 53.61 emu/g by increasing annealing temperature from 50 to 550 °C, and then decreased intensively to 0.49 emu/g after annealing at 850 °C. By increasing annealing temperature from 50 to 550, the crystallite size of NPs was increased from 6.6 to 12.7 and the coercive force (Hc) was reached to 4.20 Oe after annealing at 550 °C and then intensively increased to 1459.02 Oe for any further increasing of particle size up to 850 °C.