Structural, magnetic, vibrational and optical studies of structure transformed spinel Fe2+-Cr nano-ferrites by sintering process

Highlights: • As-synthesized cubic Fe²⁺CrFeO₄ nanoparticles transformed to hexagonal hematite with sintering temperature T. • Obtained parameters of the samples revealed effect by T and structural transition process. • Increasing T caused increase in the energy band gab (E_g). • E_g, S and revealed dependence on the crystallite size R. • Elastic wave velocities were affected by D_x, Ms and Hc, whereas Ms was affected by P. This investigation presents the optical, structural, vibrational and magnetic studies of the cubic spinel Fe²⁺CrFeO₄ nano-ferrites, which transformed to maghemite structure by annealed at 250 °C for 3 h and to hexagonal hematite at temperatures T ≥ 500 °C in air. These nanomaterials were synthesized by the co-precipitation route and sintered at different temperatures. A split of the lattice constant a to a and c occurred for T ≥ 500 °C due to the transformation to hexagonal hematite, where c/a ≈ 2.7. The density and trend of crystallite size (R) values showed increase against T, whereas the calculated and experimental specific surface area, threshold energy, Debye temperature and elastic wave velocities showed decrease. The trend of dislocation density, inter-chain distance, distortion parameter, saturation magnetization (Ms), squareness, remnant magnetization, magnetic moment and anisotropy constant was decreased with T. The main IR absorption bands; ν₁ and ν₂ of the nanomaterials were observed in the IR spectra where their positions proved dependence on T. The porosity (P), strain (), force constants, elastic and stiffness constants and coercivity revealed dependence on T. Variation of all deduced parameters with T revealed the structure transformation from cubic to hexagonal hematite. The optical band gab energy (E_g) showed increase as T increases. E_g, S and proved dependence on R, whereas Ms proved dependence on P. So, D_x, Ms and Hc proved effect on the elastic wave velocities.