Structural origin of perpendicular magnetic anisotropy in epitaxial CoPt3 nanostructures grown on WSe2(0001)

We present a detailed analysis of the local ordering in CoPt3 nanostructures epitaxially grown on WSe20001 and NaCl001 low-energy surfaces. Polarized extended x-ray absorption fine-structure measurements at the Co K-edge show a local structural anisotropy in fcc CoPt3 nanostructures grown at 300 K on WSe2. 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 L12-ordered nanostructures grown at 700 K. In contrast, the short-range order is isotropic in CoPt3 nanostructures grown on NaCl001 at 370 K. These different behaviors emphasize the favorable role of Se segregated atoms of WSe2 in the dynamic segregation of Pt atoms at the advancing surface during codeposition, which governs the local structural anisotropy. In the absence of Se, as previously observed in epitaxial CoPt3 films grown on Ru buffer layers, the development of similar structural anisotropy requires higher growth temperatures 550-720 K.