Synthesis and Physical Characterization of γ-Fe₂O₃ and (α+γ)-Fe₂O₃ Nanoparticles

Magnetic nanoparticles were synthesized at different hydrothermal temperatures (HT; 100, 130, 160 and 190 ℃) by using a facile hydrothermal route combined with a subsequent calcination process. The calcined materials were analyzed for phase, microstructure, and magnetic and dielectric properties through different characterization techniques. The structural analyses revealed that the material prepared at a HT of 100 ℃ and sequentially calcined at 300 ℃ for 3 h showed a high degree of the maghemite structure. On the other hand calcined materials showed a small additional peak belonging to the hematite structure. FESEM micrographs of the materials calcined at HT, of 100 ℃ and 190 ℃ showed spherical-like nanoparticles with diameters in range 30 - 54 nm. Materials prepared at a HT of 160 ℃ followed by calcination at 300 ℃ for 3 h exhibited the highest saturation magnetization, M_s = 67 emu/g, with a lower coercivity; all materials were in a single domain state. A high dielectric constant (105.54) was observed for the calcined material that had been prepared at a HT of 130 ℃. The dielectric properties of synthesized materials showed an almost frequency- independent behavior.