The citrate method was used successfully to synthesize rare earth erbium (Er3+) doped Co-Mn nanoferrites (CME nanoferrites) with the chemical formulation Co0.5Mn0.5ErxFe2-xO4 (0.0 ≤x ≤0.1). Specimens' X-ray diffraction (XRD) patterns ensured the production of a single-phase cubic spinel structure; although, a secondary phase of Er2O3 had been observed at higher Er concentration (x ≥0.06). The lattice parameter (a) rose as the Er3+ content in the lattice grew. Average crystallite size, determined by Williamson–Hall method, increased first up to x = 0.06 and then declined at higher values of x. According to FTIR analysis revealed that the spectra included two main absorption bands at ~ 600 and 400 cm-1, as well as other bands. The band gap was estimated using UV-Diffuse reflectance (DR) spectroscopy, which ranged between 1.39 and 1.48 eV.