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

MFeO (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 CoFeO and MnFeO, respectively. The distribution of metal cations at the tetrahedral and octahedral sites in the ABO 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 CoFeO and 2.64 eV for MnFeO. 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 (CoFeO) and 49 emu g (MnFeO) at 55 K-values that are among the highest reported for these systems; the squareness ratios were 0.58 and 0.12, respectively. The CoFeO sample exhibited high effective anisotropy due to surface spin contributions, resulting in high coercivity and squareness. In contrast, the enhanced dipolar interactions in MnFeO reduced coercivity and squareness. These magnetic behaviors were interpreted within the frameworks of the Stoner-Wohlfarth and superparamagnetic models that account for interparticle interactions.