Minority-spin impurity band in n -type (In,Fe)As: A materials perspective for ferromagnetic semiconductors

Fully understanding the properties of n-type ferromagnetic semiconductors (FMSs), complementary to the mainstream $\mathrm{p}$-type ones, is a challenging goal in semiconductor spintronics because ferromagnetism in $\mathrm{n}$-type FMSs is theoretically nontrivial. 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 $\mathrm{n}$-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.