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

We present a detailed analysis of the local ordering in ${\text{CoPt}}_{3}$ nanostructures epitaxially grown on ${\text{WSe}}_{2}(0001)$ and NaCl(001) low-energy surfaces. Polarized extended x-ray absorption fine-structure measurements at the $\text{Co}\text{ }K$-edge show a local structural anisotropy in fcc ${\text{CoPt}}_{3}$ nanostructures grown at 300 K on ${\text{WSe}}_{2}$. 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 $\text{L}{1}_{2}$-ordered nanostructures grown at 700 K. In contrast, the short-range order is isotropic in ${\text{CoPt}}_{3}$ nanostructures grown on NaCl(001) at 370 K. These different behaviors emphasize the favorable role of Se segregated atoms of ${\text{WSe}}_{2}$ 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 ${\text{CoPt}}_{3}$ films grown on Ru buffer layers, the development of similar structural anisotropy requires higher growth temperatures (550--720 K).