Layer-Resolved Antisite Mapping in MnSb2Te4 Grown by Molecular Beam Epitaxy

Magnetic topological insulators (MTIs) are attractive materials for spintronic and quantum devices. Among these, MnSb2Te4 has attracted strong interest due to its relatively high Curie temperature and its ferromagnetic ground state, which are strongly influenced by Mn-Sb cation disorder. This disorder primarily arises from antisite defects, namely, Sb occupying Mn (3a) sites and, conversely, Mn occupying Sb (6c) sites, a phenomenon that is rarely quantified with layer- and site-specific precision. Here, we grow high-quality MnSb2Te4 thin films using a digital-alloy molecular beam epitaxy method based on a short-period Sb2Te3/MnTe stacking sequence. This method enables the adjustment of the nominal Sb/Mn ratio and reliable formation of the septuple-layer phase. The resulting films are single-phase and highly crystalline. Stoichiometric films exhibit ferromagnetism with perpendicular anisotropy and TC ≈ 50-55 K, and show topological surface states by photoemission spectroscopy. We then establish a quantitative, Wyckoff-resolved antisite metrology using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) intensity profiling. Notably, even in nominally stoichiometric films, we resolve a pronounced septuple-layer-to-septuple-layer spread in local Sb/Mn ratios along the growth direction, demonstrating intrinsic compositional inhomogeneity on the septuple-layer length scale. Antisite mapping reveals strong site selectivity. Sb occupancy of the central Mn (3a) site remains approximately constant at ∼30-40% with varying stoichiometry. However, the Mn occupancy of the outer Sb (6c) sites increases with Mn content (from ∼0 to ∼50% in the most Mn-rich septuple layers). These results identify the 6c sites as the primary incorporation channel for excess Mn and provide a practical route to quantify and ultimately control cation intermixing in MTI thin films.