Abstract <p>In this study, Mn_xNi_1–xFe₂O₄ (<i>x</i> = 0 to 1) nanoparticles (MNPs) were prepared by a sol<b>–</b>gel auto-combustion route. The X-ray diffraction (XRD) analysis, Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM) measurements are used to investigate the effect of Mn substitution on the structure and magnetic properties of the samples. The XRD patterns confirmed that all the samples show a cubic spinel structure; further, the spinel structure of ferrite is validated by FTIR. As Mn²⁺ ion concentration increases, the peak of the Fe–O bond moves to a lower frequency at tetrahedral sites. Transmission electron microscopy (TEM) analysis depicted particle sizes ranging from 47.70 ± 2.6 to 56.74 ± 3.1 nm with increasing Mn²⁺ substitution. The saturation magnetization (<i>M</i>_s) increased from 26.21 to 34.95 emu/g, while the coercivity (<i>H</i>_c) remained negligible with increasing Mn²⁺ content. The self-heating characteristics of Mn_xNi_1–xFe₂O₄ (<i>x</i> = 0 to 1) MNPs in an AC magnetic field were evaluated by specific absorption rate (SAR) and intrinsic loss power, both of which were presented with varying MNPs composition and field amplitudes. The optimized Mn_0.2Ni_0.8Fe₂O₄ MNPs can be used as a promising candidate for hyperthermia applications.</p>