Antiferromagnetic (AFM) materials show significant potential for spintronic applications because of their fast spin dynamics, magnetic stability, and uncompensated magnetic moments, which can be leveraged for information storage. In this study, we investigate long-range coupling between single-crystalline ${\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}$ (FeMn) layers separated by a Cu spacer in fully epitaxial 8-monolayer (ML) FeMn/Cu/6-ML FeMn/Co/Fe/Cu(001) heterostructures. We observe a periodic modulation of the induced perpendicular magnetic anisotropy as a function of Cu spacer thickness, with a period of approximately 5.5–6 ML. In contrast, control samples [8-ML FeMn/Cu/6-ML FeMn/Co/Cu(001)], designed with in-plane magnetization and interfacial coupling, show a similar modulation phase but a weaker variation in coupling strength. These findings provide clear evidence of: (1) long-range exchange coupling between spatially separated AFM FeMn layers across the Cu spacer, and (2) a periodic modulation of long-range AFM ordering in the FeMn films, driven by this interlayer coupling as a function of Cu spacer thickness, where this behavior is primarily governed by the out-of-plane uncompensated moments of the FeMn layers, stemming from their $3Q$-type noncollinear AFM spin structure. Our results emphasize the crucial role of AFM spin structure in mediating long-range magnetic interactions, offering valuable insights for designing next-generation AFM/normal-metal-based spintronic heterostructures.

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• Antiferromagnetism
• Magnetic coupling
• RKKY interaction
• Magnetic multilayers
• Transition metals
• Ultrathin films
• Low-energy electron diffraction
• Magneto-optical Kerr effect