Antiferromagnets provide a promising route for inducing perpendicular magnetic anisotropy (PMA) in adjacent ferromagnetic (FM) layers through exchange coupling, yet the respective roles of the magnetic phase stability, antiferromagnetic (AFM) spin configuration, and lattice distortion remain incompletely understood. Here we investigate the composition-dependent magnetic phase evolution of epitaxial fcc-like ${\mathrm{Co}}_x{\mathrm{Mn}}_{1\text{−}x}$ alloy films and its impact on PMA in ${\mathrm{Co}}_x{\mathrm{Mn}}_{1\text{−}x}/\mathrm{Co}/\mathrm{Fe}/\mathrm{Cu}\left(001\right)$ heterostructures. A transition from long-range AFM order ($x\le 0.47$) to ferromagnetism ($x\ge 0.71$) is identified, with a crossover near $x\approx 0.5–0.6$ within a single crystallographic framework. Pronounced PMA develops within the AFM regime and reaches a maximum at $x=0.2$, well separated from the AFM-FM boundary. The nonmonotonic dependence of PMA on composition, despite the systematic evolution of lattice distortion, indicates that lattice distortion alone cannot account for the observed PMA behavior. Layer-selective substitution experiments further reveal a pronounced depth dependence, indicating that the induced PMA is strongly influenced by magnetic interactions associated with the AFM spin configuration within the AFM layer, rather than by interfacial modification alone. These results indicate that the AFM-FM phase transition sets the composition range for PMA, long-range AFM ordering defines its onset, and the internal AFM spin configuration governs its magnitude. These findings further suggest that fcc-like ${\mathrm{Co}}_x{\mathrm{Mn}}_{1\text{−}x}$ alloys provide a model system for systematically tuning exchange-mediated magnetic anisotropy in epitaxial AFM/FM heterostructures.

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• Antiferromagnetism
• Magnetic anisotropy
• Magnetic coupling
• Magnetic phase transitions
• Antiferromagnets
• Magnetic multilayers
• Transition metal alloys
• Ultrathin films
• Low-energy electron diffraction
• Magneto-optical Kerr effect