Substitution effect of Cr³⁺ on hyperfine interactions, magnetic and optical properties of Sr-hexaferrites

Chromium-substituted strontium nano-hexaferrites, SrCr_xFe_12-xO₁₉ (0.0 ≤ x ≤ 1.0), were produced by the hydrothermal route. The structure, morphology, optical and magnetic properties were studied. Crystal structure, spinel phase and functional groups were verified using X-ray diffraction and Fourier Transform Infrared spectroscopy. The average crystallites size is ranging between 50 and 67 nm. Scanning electron microscopy observations indicated that the multi-grains of the particles are aggleromated and are hexagonal in shape. The isotherm plots obtained from the nitrogen physisorption experiments showed that the mesopore area and the total pore volume decreased progressively with increasing the Cr³⁺ content. Besides, the band gap energy (E_g) decreases from 1.75 to 1.43 eV. The magnetic hysteresis loops showed that the produced nano-hexaferrites exhibit hard ferromagnetic-like (FM) behavior. Compared to the pristine SrFe₁₂O₁₉ product, the deduced saturation (M_s) and remanent (M_r) magnetizations, the coercivity (H_c) and the magnetocrystalline anisotropy field (H_a) increase for lower Cr³⁺ content (x ≤ 0.4) and then decrease as the Cr³⁺ content further increases. The squareness ratio M_r/M_s are ranging between 0.5 and 0.6, suggesting the single domain nano-sized particles with uniaxial anisotropy for various synthesized products. Mossbauer analysis was done to determine the quadrupole splitting, the isomer shift, and the hyperfine magnetic field values of all products.