This study investigates the effects of Zn–Mn co-substitution on the structural, morphological, magnetic, and optical properties of nickel spinel ferrite nanoparticles with formula \({\text{Ni}}_{(1-x)}{\text{Zn}}_{x}{\text{Mn}}_{y}{\text{Fe}}_{(2-y)}{\text{O}}_{4}\) , where x and y range from 0 to 0.6. The sol–gel method was used for the synthesis of nanoparticles. x-Ray diffraction (XRD) analysis confirmed the formation of a single-phase spinel structure with crystallite size decreasing from 18 nm to 7 nm as the Zn content (x) was increased and the Mn content (y) was decreased. Scanning electron microscopy (SEM) revealed irregularly shaped grains in all samples. Fourier-transform infrared (FTIR) spectroscopy showed characteristic absorption bands at 557 cm−1 and 426 cm−1, corresponding to vibrations at tetrahedral and octahedral sites, respectively. Optical bandgap energies, estimated from Tauc plots, ranged from 4.65 eV to 4.61 eV. Magnetic hysteresis (M–H) loops indicated ferrimagnetic behavior. Increasing y and decreasing x led to significant enhancement in the saturation magnetization ( \({M}_{\rm S}\) ) from 6 emu/g to 27 emu/g, while the coercivity ( \({H}_{\rm C}\) ) decreased from 650 Oe to 150 Oe. The sample with x = 0.5 and y = 0.2 exhibited the highest \({M}_{\rm S}\) value of 27 emu/g. Owing to their high \({M}_{\rm S}\) and low \({H}_{\rm C}\) , the prepared nanoparticles are well-suited for applications in ferrofluids.
