  A new group-IV ferromagnetic semiconductor, Ge1-xFex, was successfully grown
by low-temperature molecular beam epitaxy (LT-MBE) without precipitation of
ferromagnetic Ge-Fe intermetallic compounds. The ferromagnetism of Ge1-xFex
films was investigated by magnetic circular dichroism (MCD). In particular, the
influence of the Fe content (FFe/FGe =1 - 10%) and growth temperature (100,
200OC) on the ferromagnetism was carefully studied. The MCD measurements
revealed that the band structure of the Ge1-xFex films was identical with that
of bulk Ge, and that the large spin splitting of the band structure was induced
by the incorporation of Fe atoms into the Ge matrix, indicating the existence
of s,p-d exchange interactions. The Ge1-xFex films showed ferromagnetic
behavior and the ferromagnetic transition temperature linearly increased with
increasing the Fe composition. These results indicate that the epitaxially
grown Ge1-xFex is an intrinsic ferromagnetic semiconductor.
