<p>Lanthanum-doped cobalt ferrite nanoparticles with the chemical composition CoFe_2−xLa_xO₄ (x = 0.0, 0.1, 0.2, 0.3, and 0.4) were prepared via sol–gel auto-combustion technology followed by microwave sintering. The X-ray diffraction (XRD) pattern analysis confirms that all prepared samples exhibit a cubic spinel structure, whilst excess La³⁺ ions doping results in the formation of an additional phase, LaFeO₃. Scanning electron microscopy (SEM) observations revealed that all prepared samples exhibited spherical cubic morphology. The optical properties of the sample was characterised using ultraviolet–visible diffuse reflectance spectroscopy (UV–VIS-DRS) and Photoluminescence (PL). With increasing La³⁺ ions doping content, the optical bandgap of CoFe_2-xLa_xO₄ decreases slightly, ranging from 1.57 to 1.37&#xa0;eV. The PL emission spectrum study indicates the radiative defects and oxygen voids in La³⁺ ions doped CoFe₂O₄ nanoparticles present in the synthesized samples. Characterisation of the Vibrating sample magnetometer (VSM) data indicates that La³⁺ rare-earth ions doping refines the grain structure whilst simultaneously reducing both the saturation magnetisation and the remanence. However, doping with a certain amount of La³⁺ ions can enhance coercivity of samples. The reduction in magnetic moment can be attributed to the substitution of non-magnetic La³⁺ ions for Fe³⁺ ions (5 µ_B), thereby leading to a decrease in the net total magnetic moment. This also indicates that the Lanthanum doped cobalt ferrite has low magnetic properties.</p>