  For the study of ferromagnetic materials which are compatible with group-IV
semiconductor spintronics, we demonstrate control of the ferromagnetic
properties of Heusler-alloys Fe_3-xMn_xSi epitaxially grown on Ge(111) by
tuning the Mn composition x. Interestingly, we obtain L2_1-ordered structures
even for nonstoichiometric atomic compositions. The Curie temperature of the
epilayers with x ~ 0.6 exceeds 300 K. Theoretical calculations indicate that
the electronic structures of the nonstoichiometric Fe_3-xMn_xSi alloys become
half-metallic for 0.75 < x < 1.5. We discuss the possibility of
room-temperature ferromagnetic Fe_3-xMn_xSi/Ge epilayers with high spin
polarization.
