Epitaxial magneto-optical Co/Ag/Ge(001) ultrathin films were investigated at both room temperature and low temperature. variations in coercivity as a function of thickness and temperature suggest the existance of interfacial exchange interactions. The operative layers required to inhibit intermixing of Co-Ge alloys from the oscillation of coercivity with Ag film thickness were determined. When the film thickness was reduced to the submonolayer range, perpendicular anisotropy was unexpectedly found. Thus unusual phenomenon can qualitatively discussed in terms of effective magnetic anisotropy by the contribution of reduced Co layers and by surface magnetic anisotropy. (c) 2007 Elsevier B.V. All rights reserved.