<p>Nanocrystalline ferrites consisting of magnesium and zinc, represented by the formula Mg_0.2Zn_0.8TbxFe_2−xO₄ (where x varies from 0.00 to 0.25), were synthesized using the sol-gel auto-combustion method. Their structural and microstructural characteristics were analyzed using X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM). X-ray diffraction analysis indicated that the undoped sample exhibited a single-phase cubic spinel structure, whereas the Tb³⁺-doped samples demonstrated both a predominant spinel phase and a secondary orthorhombic phase. An increase in the concentration of Tb³⁺ ions resulted in an increase in crystallite size, accompanied by a decrease in the lattice parameters of the samples. This phenomenon was attributed to the substitution of Fe³⁺ ions with Tb³⁺ ions in the crystal lattice. Furthermore, the incorporation of Tb³⁺ ions increased both the X-ray density and porosity of the samples. Fourier-transform infrared (FT-IR) spectroscopy confirmed the presence of cubic spinel ferrites by identifying two metallic bands within the range of 200–650&#xa0;cm^− 1. UV-visible analysis indicated that the direct bandgap energy was within the expected range, 450–560&#xa0;nm. The temperature-dependent electrical properties verified the semiconducting nature of these samples. VSM analysis indicated soft magnetic behavior, with the substitution of Tb³⁺ ions resulting in a decrease in all the magnetic properties. An in vitro cytotoxicity investigation of MCF-7 breast cancer cells showed a dose-dependent decline in cell viability. Importantly, moderate levels of Tb³⁺ ions achieved an optimal balance between cytotoxicity and cell biocompatibility. These results underscore the potential utility of these materials in biomedical applications, particularly in magnetic hyperthermia therapy.</p>