<p>This research focuses on the phase transformation of lead hexagonal ferrite (PbFe₁₂O₁₉) to cubic spinel ferrite (CoFe₂O₄) induced by increasing the substitution ratio of Co²⁺ ions. Lead cobalt ferrite nanoparticles with varying Co²⁺ content (Pb_1-xCo_xFe₁₂O₁₉, where <i>x</i> = 0.0, 0.25, 0.50, 0.75, and 1.0) were synthesized using the sol-gel auto-combustion method. The structural properties of these nanoparticles were investigated through X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and high-resolution transmission electron microscopy (HRTEM). Magnetic characteristics were determined using vibration sample magnetometry (VSM). XRD analysis confirmed the formation of PbFe₁₂O₁₉ in a hexagonal phase with the P63/mmc space group. As the Co²⁺ ion concentration increased, a complete transformation to a cubic spinel structure with the Fd3m space group occurred. HRTEM observations revealed a change in the polycrystalline structure from hexagonal to cubic as the cobalt content increased at the expense of lead. Selected area electron diffraction (SAED) patterns supported the polycrystalline nature of all samples. Magnetic measurements indicated soft ferrite behavior with negligible hysteresis for all nanoparticles. The saturation magnetization (<i>M</i>_s) improved from 1.782 emu/g to 69.676 emu/g up to <i>x</i> = 1.0 of Co²⁺ ion. Remanence (<i>M</i>_r) increased from 0.269 to 34.726 emu/g as Co²⁺ ion concentration rose from 0.0 to 1.0. The squareness ratio (<i>M</i>_r/<i>M</i>_s) increased from 0.151 to 0.498 as the Co²⁺ ion concentration rose to 1.0. The tunable magnetic properties of these lead cobalt ferrite nanoparticles offer promising applications such as microwave devices, magnetic recording media, and electromagnetic wave absorbers.</p> Graphical Abstract <p></p>