Origin of robust nanoscale ferromagnetism in Fe-doped Ge revealed by angle-resolved photoemission spectroscopy and first-principles calculation

Ge1-xFex (Ge:Fe) shows ferromagnetic behavior up to a relatively high temperature of 210 K and hence is a promising material for spintronic applications compatible with Si technology. Unlike the prototypical system (Ga,Mn)As where itinerant holes induce long-range ferromagnetic order of the Mn spins, however, its ferromagnetism evolves from robust nanoscale ferromagnetic domains formed in Fe-rich regions. We have studied its underlying electronic structure by soft x-ray angle-resolved photoemission spectroscopy measurements and first-principles supercell calculation. We observed finite Fe 3d components in the states at the Fermi level (EF) in a wide region of momentum space, and the EF was located ∼0.35 eV above the valence-band maximum of the host Ge. Our calculation indicates that the EF is also within the deep acceptor-level impurity band induced by the strong p-d(t2) hybridization. We conclude that the additional minority-spin d(e) electron characteristic of the Fe2+ state is responsible for the short-range ferromagnetic coupling between Fe atoms, making the magnetism markedly different from that of (Ga,Mn)As.