  Fully understanding the properties of n-type ferromagnetic semiconductors
(FMSs), complementary to the mainstream p-type ones, is a challenging goal in
semiconductor spintronics because ferromagnetism in n-type FMSs is
theoretically non-trivial. Soft-x-ray angle-resolved photoemission spectroscopy
(SX-ARPES) is a powerful approach to examine the mechanism of carrier-induced
ferromagnetism in FMSs. Here our SX-ARPES study on the prototypical n-type FMS
(In,Fe)As reveals the entire band structure including the Fe-3d impurity bands
(IBs) and the host InAs ones, and provides direct evidence for electron
occupation of the InAs-derived conduction band (CB). A minority-spin Fe-3d IB
is found to be located just below the conduction-band minimum (CBM). The IB is
formed by the hybridization of the unoccupied Fe-3d states with the occupied
CBM of InAs in a spin-dependent way, resulting in the large spin polarization
of CB. The band structure with the IB is varied with band filling, which cannot
be explained by the rigid-band picture, suggesting a unified picture for
realization of carrier-induced ferromagnetism in FMS materials.
