<p>This study investigates the effect of annealing temperature on the structural, morphological, and magnetic properties of Zn_0.2 Mg_0.8Fe₂O₄ spinel ferrite nanoparticles synthesized via Sol-gel auto-combustion. X-ray diffraction (XRD) confirmed a single-phase cubic spinel structure with crystallite sizes increasing from 21.0 to 42.8 nm as annealing temperature rose from 400 °C to 800 °C. Fourier-transform infrared (FTIR) spectroscopy revealed characteristic Fe–O stretching vibrations at ~538 cm⁻¹ (tetrahedral site) and~ 440 cm⁻¹ (octahedral site), with reduced O–H intensity at higher temperatures, indicating improved phase purity. Field-emission scanning electron microscopy (FESEM) showed agglomerated, porous grains with average particle sizes of 144.1–171.7 nm, significantly larger than XRD-derived crystallite sizes, confirming polycrystalline agglomeration. Energy-dispersive X-ray spectroscopy (EDX) confirmed the presence of Zn, Mg, Fe, and O with stoichiometric atomic percentages. Vibrating sample magnetometry (VSM) revealed ferromagnetic behavior with saturation magnetization (M_s) increasing from 19.39 to 50.20 emu/g, attributed to enhanced crystallinity, reduced defects, and domain wall mobility. These findings highlight the critical role of annealing in tailoring the functional properties of Mg–Zn ferrites for applications in gas sensing, EMI shielding, and biomedical devices.</p> Graphical Abstract <p></p>