AlNx/DyFeCo films were deposited on glass substrates by magnetron sputtering and reactive rf magnetron sputtering. The influence of AlNx coverlayers on DyFeCo magneto-optical media was studied. The results show that nitrogen surplus in AlNx leads to changes in the MO behavior of DyFeCo films due to the reaction of Dy with nitrogen. Furthermore, the influence of thickness of AlN films on the coercivity, anisotropy and eigenvalue of Kerr rotation angle of RE-TM films was investigated. The results can be explained based on the internal stress, impurities and the pinning of defects induced by 'peening effects' of high-speed atoms.