Microwave-assisted co-precipitation synthesis of MFe₂O₄ nanoferrites (M = Co and Mn) using biogenic coir extract and...

MFe₂O₄ (M = Co and Mn) nanoparticles were synthesized from coconut coir extract using a microwave-assisted co-precipitation method, representing a green and sustainable approach for ferrite nanomaterial preparation. The physical properties of the samples were characterized using X-ray diffraction, scanning electron microscopy, ultraviolet-visible spectroscopy, photoluminescence, Raman spectroscopy, and vibrating sample magnetometry. Scanning electron micrographs revealed nanoscale morphology with evidence of polymorphism. Rietveld refinement confirmed the formation of single-phase spinel ferrites with lattice constants ranging from 8.4224 Å to 8.4782 Å for CoFe₂O₄ and MnFe₂O₄, respectively. The distribution of metal cations at the tetrahedral and octahedral sites in the AB₂O₄ spinel lattice was found to depend on the synthesis route and significantly influenced the magnetic and optical behaviors of the materials. Raman spectra exhibited characteristic peaks corresponding to a mixed spinel structure. The optical band gaps estimated from the UV-vis spectra were 2.66 eV for CoFe₂O₄ and 2.64 eV for MnFe₂O₄. PL spectra showed four distinct emission peaks at 458, 692, 758, and 871 nm. Based on UV-vis and photoluminescence spectral results, a schematic energy band structure was constructed. Magnetic measurements, analyzed using the "law of approach" to saturation, revealed saturation magnetizations of 70 emu g^-1 (CoFe₂O₄) and 49 emu g^-1 (MnFe₂O₄) at 55 K-values that are among the highest reported for these systems; the squareness ratios were 0.58 and 0.12, respectively. The CoFe₂O₄ sample exhibited high effective anisotropy due to surface spin contributions, resulting in high coercivity and squareness. In contrast, the enhanced dipolar interactions in MnFe₂O₄ reduced coercivity and squareness. These magnetic behaviors were interpreted within the frameworks of the Stoner-Wohlfarth and superparamagnetic models that account for interparticle interactions.