Effect of Annealing Temperature on Rietveld Structure Refinement, Cations Distribution, Optical, Magnetic, and Antimicrobial Activity Properties of CoFe 2 O 4 and MgFe 2 O 4 Nanoparticles Synthesized by Sol-Gel Auto Combustion Method

Ferrite nanostructures of the type MFe 2 O 4 (M = Co and Mg) were synthesized using the sol–gel auto-combustion method and subsequently annealed at 500 °C and 800 °C to examine the effect of thermal treatment on their physical and functional properties. Structural analysis performed using X-ray diffraction and Rietveld refinement confirmed the formation of a single-phase spinel structure for all samples. The crystallite size, estimated using the Debye–Scherrer and Williamson-Hall methods, was found to increase with annealing temperature, which was further supported by field-emission scanning electron microscopy observations showing grain growth. Optical studies based on UV–Visible absorption spectra revealed an increase in the optical band gap with increasing annealing temperature. Magnetic measurements carried out at room temperature showed enhanced magnetic properties for cobalt ferrites, whereas magnesium ferrites exhibited a reduction in magnetic behavior upon annealing. In addition, antibacterial activity tests against both Gram-positive and Gram-negative bacterial strains demonstrated that thermal treatment significantly influences the antimicrobial performance of the ferrite nanostructures. Overall, the results highlight the critical role of annealing temperature in tailoring the structural, optical, magnetic, and antibacterial properties of ferrite materials for potential multifunctional applications.