  We theoretically investigate the possibility of establishing ferromagnetism
in the topological insulator Bi2Se3 via magnetic doping of 3d transition metal
elements. The formation energies, charge states, band structures, and magnetic
properties of doped Bi2Se3 are studied using first-principles calculations
within density functional theory. Our results show that Bi substitutional sites
are energetically more favorable than interstitial sites for single impurities.
Detailed electronic structure analysis reveals that Cr and Fe doped materials
are still insulating in the bulk but the intrinsic band gap of Bi2Se3 is
substantially reduced due to the strong hybridization between the d states of
the dopants and the p states of the neighboring Se atoms. The calculated
magnetic coupling suggests that Cr doped Bi2Se3 is possible to be both
ferromagnetic and insulating, while Fe doped Bi2Se3 tends to be weakly
antiferromagnetic.
