Tunability of Bandgap and Magnetism in K and Pb Codoped BiFeO₃ Nanoparticles for Multiferroic Applications: The Role of Structural Transition and Fe Deficiency

BiFeO3 is the most well-known single-phase material for multiferroic and photovoltaic applications because of its large ferroelectric polarization and high operating temperature. However, direct magnetoelectric coupling between ferroelectricity and magnetic order in bulk BiFeO3 is exceedingly weak, resulting from its intrinsic cycloidal spin structure. Fortunately, nanostructured BiFeO3 could exhibit multiferroism due to superficial uncompensated spins, thereby altering the magnetic order of the bulk phase. Here, the optical property and magnetism of Bi1–x(KPb)xFeO3 (0 ≤ x ≤ 0.1) nanoparticles are studied. X-ray powder diffraction and Raman spectroscopy reveal the apparent increase of cubic crystal phase (Pm-3m) and the emergence of Fe deficiency with the increase of K and Pb codoping content in BiFeO3 nanoparticles. Fe deficiency and its change in Bi1–x(KPb)xFeO3 nanoparticles are further confirmed by EDX and XPS spectra, besides the presence of Bi3+, Fe3+, and Pb4+ ions. The optical bandgap is narrowed gradually in the samples with the increase of K and Pb codoping content, and the corresponding phenomenological model is proposed. Magnetic hysteresis and magnetization versus temperature curves indicate the coexistence of antiferromagnetic and ferromagnetic orderings in Bi1–x(KPb)xFeO3 nanoparticles. The saturation magnetization first decreases and then increases with K and Pb codoping concentration increasing. A correlation among the structural transition, Fe deficiency, and magnetic property has been established in detail, which provides a possibility to further understand the cycloidal spin magnetic structure of BiFeO3 and improve its magnetic property for multiferroic applications.