The structural and magnetic properties of BaFe₁₂O₁₉ nanoparticles: effect of residual sodium ions

We studied the structural and magnetic properties of BaFe₁₂O₁₉ (BaM) nanoparticles synthesized by a co-precipitation route based on a modified citrate process. The lattice contraction of co-precipitated BaM nanoparticles is detected after prolonged sintering at 900 °C. In addition, investigation with X-ray photoelectron spectroscopy (XPS) provides evidence of a highly enhanced population of oxygen vacancies. The lattice distortion and formation of oxygen vacancies are attributed to a direct result of substitutional incorporation of smaller Na ions into Ba²⁺ sites in the lattice of BaM during prolonged annealing. The aliovalent impurities are assumed to be originated from NaOH which has been incorporated as a pH modifier. Magnon scattering is detected at 1640 cm^-1 in the low temperature Raman spectra of BaM. Minimization of the magnon peak is also identified after prolonged annealing, which indicates oxygen vacancy-induced collapse of strong anti-ferromagnetic interaction between Fe³⁺ ions in the bipyramidal sites and the octahedral sites in BaM nanoparticles. As a result, the saturation magnetization (M_s) of BaM nanoparticles is enhanced by the onset of local ferromagnetic interaction induced by the collapse of antiferromagnetic interaction between oppositely aligned spins. In this study, we re-investigated the evolution of structural and magnetic properties with prolonged annealing of BaM nanoparticles and the effects of residual Na ions are also discussed.