¹ Department of Physics, College of Education, Mustansiriya University, Baghdad, Iraq;

² College of Education for Pure Science, Ibn Al Haitham, University of Baghdad, Baghdad, Iraq.

* Correspondence: taghreed.m.m@ihcoedu.uobaghdad.edu.iq

In this research, Co_xCu_0.2Zn_0.8-xFe₂O₄ nanoferrites (x = 0.0, 0.03, 0.06, 0.09, and 0.12) were prepared through the sol–gel method, and the effects of cobalt substitution on their structural and microscopic properties were investigated. X-ray diffraction (XRD) results revealed the formation of a pure cubic spinel phase without secondary phases, with the lattice constant gradually decreasing as the cobalt concentration increased. Crystal size, strain, and stress were analyzed by employing the Scherrer equation and three Williamson–Hall model models (uniform deformation, uniform stress, and uniform deformation energy density models). Results showed that extracted values varied depending on the model used because the models had different physical assumptions related to the strain and stress within the crystal lattice. Field-emission scanning electron microscopy (FESEM) images revealed partial agglomeration, nonuniform grain distribution, and grain sizes ranging from 65 ± 5 nm to 83 ± 10 nm. The crystal size inferred from XRD clearly differed from the grain size inferred from FESEM, confirming the polycrystalline nature of the prepared samples. The results of this study indicate that cobalt substitution directly affects the structural and microscopic properties of the prepared system.