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We report on anomalous antiferromagnetic coupling in Fe/Si/Fe epitaxial structures with interfacial cobalt “dusting”. Antiferromagnetic coupling exceeds 75 μJ/m2 for a 2.0 nm-thick Si spacer and is still detectable for a 2.3 nm-thick spacer at room temperature. From room temperature to T**∼150K the magnetization follows Bloch's law and the coupling shows metallic-type behaviour; for lower temperatures the coupling is of insulating type. Below T*∼50K an additional magnetic signal arises and the coupling becomes metallic-type again. We relate the observed features to the formation of inter-diffused magnetic FeCo-silicides with a Curie temperature close to 50 K.