MnFe2O4 nanoparticles of different crystallite size were prepared by varying the different concentrations of Mg2+ dopant ions using microwave assisted combustion method. The structural properties of the ferrite and modified ferrite were characterized by XRD (X-ray powder diffraction), Rietveld analysis, FT-IR (Fourier transforms infra red spectroscopy), HR-SEM (high resolution scanning electron microscopy), UV–Visible spectroscopy, PL (photoluminescence spectroscopy), and VSM (vibrating sample magnetometer). The concentration of the dopant metal ions plays an important role in phase, purity, morphology, optical and magnetic properties. X-ray diffraction and the HR-SEM analysis indicate that all the samples are single phase, crystalline and homogeneous in nature. The presence of metal oxides is confirmed by FT-IR analysis. The band gap energy of the samples has been studied using UV–Visible DRS measurements and the effect of Mg2+ doping on absorption spectra is also investigated. The band gap energy of the sample decreases with increasing the concentration of the dopant ions. The PL spectrum shows the emission peak at 565 nm with an excitation wavelength of 414 nm. The considerable decrease in saturation magnetization and increase in the coercivity value is determined using vibrating sample magnetometer. © 2017, Springer Science+Business Media New York.