A Strategy for Tuning the Structure, Morphology, and Magnetic Properties of MnFeO/SiO Ceramic Nanocomposites via Mono-, Di-, and Trivalent Metal Ion Doping and Annealing.

This work presents the effect of monovalent (Ag, Na), divalent (Ca, Cd), and trivalent (La) metal ion doping and annealing temperature (500, 800, and 1200 °C) on the structure, morphology, and magnetic properties of MnFeO/SiO ceramic nanocomposites synthesized via sol-gel method. Fourier-transform infrared spectroscopy confirms the embedding of undoped and doped MnFeO nanoparticles in the SiO matrix at all annealing temperatures. In all cases, the X-ray diffraction (XRD) confirms the formation of MnFeO. In the case of undoped, di-, and trivalent metal-ion-doped gels annealed at 1200 °C, three crystalline phases (cristobalite, quartz, and tridymite) belonging to the SiO matrix are observed. Doping with mono- and trivalent ions enhances the nanocomposite's structure by forming single-phase MnFeO at low annealing temperatures (500 and 800 °C), while doping with divalent ions and high annealing temperature (1200 °C) results in additional crystalline phases. Atomic force microscopy (AFM) reveals spherical ferrite particles coated by an amorphous layer. The AFM images showed spherical particles formed due to the thermal treatment. The structural parameters calculated by XRD (crystallite size, crystallinity, lattice constant, unit cell volume, hopping length, density, and porosity) and AFM (particle size, powder surface area, and thickness of coating layer), as well as the magnetic parameters (saturation magnetization, remanent magnetization, coercivity, and anisotropy constant), are contingent on the doping ion and annealing temperature. By doping, the saturation magnetization and magnetocrystalline anisotropy decrease for gels annealed at 800 °C, but increase for gels annealed at 1200 °C, while the remanent magnetization and coercivity decrease by doping at both annealing temperatures (800 and 1200 °C).