CeO 2 and CeO 2 -based materials have attracted extensive attention because of their good chemical stability, good dielectric properties, high refractive index, high visible and infrared transmittance, and high ultraviolet radiation absorption efficiency. It can be widely used in optical coatings, microelectronics, solidstate electrolyte for fuel cells (SOFCs), and other optoelectronic devices. [1] Recently, the preparation of CeO 2 -based diluted magnetic semiconductors (DMS) by transition metal (TM) doping has also attracted researchers' attention. [2,3] DMS can control the charge and spin of electrons at the same time. [4] Therefore, it has potential applications in spin-valve transistors, spin light-emitting diodes, nonvolatile storage, and logic devices. [5][6][7] In the field of optoelectronic applications, DMS is also concerned by the majority of scientific and technological workers. The doping of TM in CeO 2 can change its bandgap, thus changing its optical, electrical, and magnetic properties. [8] Generally, TM-doped CeO 2 can change its optical bandgap and optical properties in a large range, which is helpful for optoelectronic device engineering in a wide spectral range. The doping of TM ions such as Cu, Cr, Co, Mn, Fe, and Ni in CeO 2 has been reported, and its optoelectronic properties have attracted special attention. [9][10][11] In addition, many experimental and theoretical studies show that TM (such as Co)-doped CeO 2 is a potential DMS with room-temperature ferromagnetism, [12] because it has appropriate position occupancy and vacancy defects caused by doping. Its magnetic properties can be adjusted by changing the doping concentration. Many research groups believe that room-temperature ferromagnetism in DMS materials prepared by TM-doped CeO 2 is caused by the oxygen vacancy mechanism, [13,14] but its magnetic origin is still being explored. In addition, the optical properties of TM (such as Co)-doped CeO 2 films are also very worthy of study, because the detailed study of the optical properties of these materials and their dependence on preparation methods and conditions is the basic knowledge of the application of these materials in the field of optics and optoelectronic devices.The preparation methods of TM (such as Co)-doped CeO 2 films have been reported in the literature. [15][16][17] Among these preparation methods, radio frequency (RF) magnetron sputtering has attracted special attention, because it has the advantages of easy control of film growth, good repeatability, low deposition temperature, good stability and uniformity, and ease of preparation of films on a large scale. Therefore, Co-doped CeO 2 thin films will be prepared by RF magnetron sputtering in this work. It is well known that the properties of RF-sputtered Co-doped CeO 2 films are related to RF power, pressure, substrate temperature, ambient atmosphere, and post-treatment process after deposition. [18] For example, the presence of different orientations