  We investigated the origin of ferromagnetism in epitaxially grown Mn-doped Ge
thin films. Using low-temperature molecular beam epitaxy, Mn-doped Ge films
were successfully grown without precipitation of ferromagnetic Ge-Mn
intermetallic compounds, such as Mn5Ge3. Magnetic circular dichroism
measurements revealed that the epitaxially grown Mn-doped Ge films exhibited
clear ferromagnetic behavior, but the Zeeman splitting observed at the critical
points was not induced by the s,p-d exchange interactions. High-resolution
transmission electron microscopy and energy dispersive X-ray spectroscopy
analyses show phase separation of amorphous Ge1-xMnx clusters with high Mn
content from a Mn-free monocrystalline Ge matrix. Since amorphous Ge1-xMnx was
characterized as a homogeneous ferromagnetic semiconductor, the precipitation
of the amorphous Ge1-xMnx clusters is the origin of the ferromagnetic
semiconductor behavior of the epitaxially grown Mn-doped Ge films.
