Chemical substitution-induced structure transition and enhanced magnetic and optical properties of sol–gel synthesized multiferroic BiFeO₃ nanoparticles

Substitution-induced modifications in structural, magnetic and optical properties of Nb doped and Sm–Nb co-doped BiFeO₃ nanoparticles are reported. X-ray diffraction and Raman analyses indicate that the phase transforms from a rhombohedral to orthorhombic one due to Nb doping and Sm–Nb co-doping. The Nb doping and Sm–Nb co-doping both can lead to a decrease in particle size and an enhancement in magnetic properties, but there is a much stronger effect for the Sm–Nb co-doping than for the Nb doping. With increasing doping concentration of Nb and Sm, the maximum and remanent magnetizations first increase and then drop down, i.e., there is a maximum value in both maximum and remanent magnetizations. The optimal maximum and remanent magnetizations are obtained as 0.601 and 0.155 emu/g, respectively, at y = 0.05 for BiFe_1 - y Nb _y O₃ nanoparticles, while they are 0.954 and 0.206 emu/g at x = 0.15 for Bi_1 - x Sm _x Fe_0.95Nb_0.05O₃ nanoparticles. Meanwhile, the corresponding optical energy band gap of these samples decreases from 2.16 to 2.02 eV with increasing Sm–Nb co-doping concentration, indicating a strong absorption of visible light. Consequently, the magnetic and optical properties of BiFeO₃ nanoparticles can be improved considerably by doping high-valence transition and rare earth elements.