Role of ${\hbox {Bi}}_{2}{\hbox {O}}_{3}$ Additives on the Microstructure Development and Magnetic Properties of NiCuZn-Tb Ferrites

The role of Bi₂O₃ addition on the structural and magnetic properties of NiCuZn-Tb ferrites with composition Ni_0.5Cu_0.25Zn_0.25Tb_0.1Fe_1.9O₄+xBi₂O₃ has been investigated, where values of x (in wt %) are 0, 1, 2 and 3. Samples were prepared by the sol-gel auto combustion synthesis. The crystallographic, structural and magnetic properties of the samples were investigated by X-ray diffraction (XRD), scanning electron micrography, vibrating sample magnetometer and permeability measurements. Cubic spinel structure was observed from the XRD pattern for all the samples, implying that the Bi₂O₃ additives did not form a visible second phase in ≤ 2 wt% of Bi₂O₃. Microstructural studies showed that Bi₂O₃ forms a liquid-like phase and promotes the grain growth. The average grain size increased for ≤ 2 and then decreased for > 2 wt% of Bi₂O₃ content. The maximal grain size appears with 2 wt% Bi₂O₃. These characteristics evidently affect the magnetic properties, such as saturation magnetization (Ms) and permeability of the NiCuZn-Tb ferrites. Bi₂O₃ increase the Ms, bulk density (d_B) and permeability effectively, while excessive Bi₂O₃ concentration results in the abnormal grain growth and decrease of Ms and d_B.