We study epitaxial growth and magnetic properties of ferromagnetic alloys, Fe3Si and CoFe, on low-temperature-grown germanium (LT-Ge) layers. Despite high-quality epitaxial growth of Fe3Si and CoFe layers on Ge substrates, the structural and magnetic characteristics of Fe3Si layers are significantly degraded only when the growth is conducted on LT-Ge. We clarify that the degradation is induced by the diffusion of Ge atoms into Fe3Si. Since high-quality CoFe layers can be obtained even on LT-Ge, there is no Ge diffusion into CoFe. It is important for vertical Ge spintronics devices to understand the mechanism of the suppression of the Ge diffusion into the ferromagnetic materials.