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Amorphous Tb/Co compositionally modulated films were deposited on Si substrates with different modulation layer thicknesses ranging from about 2.5 to 15 Å. The nominal Tb to Co layer thickness ratios were systematically varied and the complex refractive index (n and k) and polar magneto-optical Kerr effects (rotation and ellipticity) were measured in the 3000–8000-Å spectral range as well. The samples were divided into two groups. In one group, the thickness of the Co layers was fixed, the Tb layer thickness varied. In the second group, the thickness of the Tb layer was fixed, and that of the Co layer thickness varied. The Kerr rotation and the coercivities of the samples showed very consistent and interesting changes. X-ray diffraction and x-ray fluorescence were also performed on the samples, which revealed layered structures, or compositional modulation, and provided information on the Tb to Co atomic ratios in the samples. Journal of Applied Physics is copyrighted by The American Institute of Physics.