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The Mn _5 Ge _3 compound, thanks to its room-temperature ferromagnetism, metallic character and ability to epitaxially grow on germanium, acts as a potential candidate for spin injection into group-IV semiconductors. Understanding and controlling Ge overgrowth behaviour on Mn _5 Ge _3 /Ge heterostructures represents a crucial step to realize Ge/Mn _5 Ge _3 /Ge multilayers for numerous spintronic applications. Here, we have combined structural and morphological characterizations with magnetic analyses to study the mechanisms of Ge overgrowth on epitaxial Mn _5 Ge _3 layers in the temperature range of 450–550 °C. It is found that deposited Ge instantly reacts with Mn to form a Mn _5 Ge _3 surface layer, which, acting as a surfactant, continuously floats upwards from the growing surface to a distance larger than 70 nm. New Ge layers are successively formed underneath, allowing such a floating Mn _5 Ge _3 surface layer to be stabilized by epitaxy. These observations can be considered as a typical example in which the stabilization of metastable thin films by epitaxy can overcome thermodynamic equilibrium. We have also investigated the effect of carbon adsorption on the top of the Mn _5 Ge _3 layer prior to Ge deposition to control the Mn:Ge reaction. It is shown that adsorbed carbon effectively reduces the out-diffusion of Mn from Mn _5 Ge _3 , allowing Ge layers to stack up on top of Mn _5 Ge _3 . However, at temperatures of 450–550 °C, carbon may react with Mn to form manganese carbides and the resulting Ge overlayers are found to change their orientation from the (111) plane to the (001) plane, which has a higher surface energy. Finally, a strategy to realize Ge/Mn _5 Ge _3 /Ge multilayers will be addressed.