LAUSR

Abstract Nano-sized Bi 2 Fe 4 O 9 (BFO) was successfully synthesized using a new reverse chemical co-precipitation method at different pH values of 8–12. These powders were examined by x-ray diffractometery (XRD), thermogravimetrical differential thermal analysis (TG-DTA), field emission scanning electron microscopy (FESEM) and vibrating sample magnetometery (VSM). The XRD analysis showed the formation of pure phase Bi 2 Fe 4 O 9 at calcination temperature over 700 ℃. The TG-DTA curves indicated the crystallization temperature of 617 ℃ for the Bi 2 Fe 4 O 9 sample. The FESEM micrographs revealed a precipitates agglomeration, which is related to the nature of the chemical co-precipitation method and free surface energy of nanoparticles. Furthermore, the particle size of the powder samples increased from 43 to 131 nm as the pH value increased from 8 to 12, respectively. Also, the morphological change from nearly cubic to rod-like shape in the nanoparticles was observed by increasing the pH value. The M-H curves of the as-prepared powders confirmed the antiferromagnetic behavior in all samples. Uncompensated spins from the surface led to the appearance of saturation magnetization in the Bi 2 Fe 4 O 9 nanoparticles. Besides, a decrease in the particles size resulted in more uncompensated spins, thereby improving the saturation and remnant magnetization from M s = 0.35 emu/g and M r = 0.010 emu/g for pH = 12 to M s = 1.15 emu/g and M r = 0.042 emu/g for pH = 8. Furthermore, as the pH values increase the coercive fields firstly rise up to 196 Oe for pH = 9 and then decrease to 151 for pH = 12.