Cation-Substitution-Driven Optoelectronic and Magnetic Tuning in NiₓMn₁₋ₓFe₂O₄ Spinel Ferrite Nanoparticles Synthesized by Co-precipitation

Abstract

Spinel ferrite nanoparticles are oxides whose structural, optical and magnetic responses can be tuned through cation substitution. This study aimed to evaluate how Ni substitution modifies the structure–property coupling of manganese ferrite nanoparticles with compositions of NiₓMn₁₋ₓFe₂O₄ (x = 0.0, 0.3, 0.5, 0.7 and 1.0). The nanoparticles were synthesised by alkaline co-precipitation, calcined at 700 °C and examined using structural, morphological, compositional, optical, photoluminescent and magnetic analyses. X-ray diffraction confirmed cubic spinel ferrite formation, with crystallite sizes of 18.59–30.44 nm. Ni incorporation contracted the lattice parameter from 8.3417 to 8.3103 Å and increased the X-ray density from 5.278 to 5.425 g cm⁻³, consistent with Mn replacement by Ni and increased formula mass. FTIR spectra showed metal–oxygen bands, with the tetrahedral band shifting from 520 to 553 cm⁻¹, supporting spinel bond rearrangement. SEM showed agglomerated quasi-spherical nanoparticles of 51–66 nm, whereas EDX confirmed cation-normalised compositions close to target stoichiometry. UV–visible analysis indicated tunable optical behaviour, with direct band gaps of 2.07–2.69 eV and Urbach energies of 0.189–0.643 eV. PL spectra displayed near-UV/blue emission at 377.0–383.5 nm and defect-related emission near 566–574 nm; Ni substitution quenched the main PL intensity, indicating modified recombination kinetics. VSM analysis revealed ferrimagnetic soft-magnetic behaviour, with saturation magnetisation increasing from 0.0073 to 0.7723 emu, whereas coercivity decreased sharply in intermediate compositions. Integrated analysis identified NiFe₂O₄ as the most magnetically robust composition and x = 0.7 as balanced, with high optical conductivity, defect activity and soft magnetic response. Thus, Ni substitution tunes Mn ferrite nanoparticles for optoelectronic, photocatalytic, sensing and magnetic applications.