We present a detailed analysis of the local ordering in CoPt 3 nanostructures epitaxially grown on WSe 2 (0001) and NaCl (001) low-energy surfaces. Polarized extended x-ray absorption fine-structure measurements at the Co K-edge show a local structural anisotropy in fcc CoPt 3 nanostructures grown at 300 K on WSe 2. It is characterized by preferential Co-Co bonding along the in-plane direction balanced with preferential heteroatomic bonding along the out-of-plane direction and explains the unexpected perpendicular magnetic anisotropy. Such anisotropy almost vanishes in partially L 1 2-ordered nanostructures grown at 700 K. In contrast, the short-range order is isotropic in CoPt 3 nanostructures grown on NaCl (001) at 370 K. These different behaviors emphasize the favorable role of Se segregated atoms of WSe 2 in the dynamic segregation of Pt atoms at the advancing surface during codeposition, which …