Heat triggered structural transformation of SiO2 coated akaganeite nanoparticles: ε-Fe2O3 phase formation

Orthorhombic epsilon iron oxide polymorph ε-Fe₂O₃ still attracts extensive research interest due to its interesting magnetic properties and potential applications in high-density data storage and high-frequency millimeter electromagnetic waves shielding. The present paper reports on the fabrication of ε-Fe₂O₃ polymorph through thermal treatment of silica coated akaganeite nanoparticles using various heating protocols. Morphology, structure and magnetic properties of the resulting samples were assessed by various experimental techniques. Transmission electron microscopy (TEM) of the four samples calcined at temperatures above 900 °C showed that the obtained particles were roughly oval in shape with average particle diameters (D_TEM), in the range from 13 nm to 23 nm. The X-ray diffractometry confirmed presence of prevailing ε-Fe₂O₃ phase with addition of α-Fe₂O₃ phase whose amount depended on the heating regime used. The saturation magnetization, M_s, values were in the range 9.5–11.6 Am²/kg, while the room temperature (RT) coercivity fields, H_c, ranged from 111.4 kA/m (1.4 kOe) to 1002.7 kA/m (12.6 kOe). The highest M_s and H_c were achieved in the sample heated up to 930 °C through several dwelling steps at different temperatures, while another sample also annealed at 930 °C but using different heating path, displayed substantially lower coercivity H_c = 628.7 kA/m (7.9 kOe). Annealing at higher final temperatures 940 °C and 1000 °C also did not improve coercivity due to increased number of superparamagnetic particles. Presented results emphasize importance of different parameters involved in the process of thermal treatment of epsilon iron oxide precursor samples. It was demonstrated that magnetic properties of the ε-Fe₂O₃ polymorph prepared from silica coated akaganeite nanoparticles can be tuned by employing a multistep heating protocol with carefully chosen dwelling temperatures and times.