Defect-Induced Intrinsic Magnetism in Wide-Gap III Nitrides

Cation-vacancy induced intrinsic magnetism in GaN and BN is investigated by employing density-functional theory based electronic structure methods. The strong localization of defect states favors spontaneous spin polarization and local moment formation. A neutral cation vacancy in GaN or BN leads to the formation of a net moment of $3{\mu }_B$ with a spin-polarization energy of about 0.5 eV at the low density limit. The extended tails of defect wave functions, on the other hand, mediate surprisingly long-range magnetic interactions between the defect-induced moments. This duality of defect states suggests the existence of defect-induced or mediated collective magnetism in these otherwise nonmagnetic $sp$ systems.

Figure 1

Spin-resolved density of states of a Ga vacancy in a 64-atom GaN supercell. The strong spin polarization results in a complete separation between the majority- and minority-spin $t_2$ states. The nitrogen $2s$ states are not shown.

Figure 2

Isosurface charge density plot (${\rho }^0=8.5\times {10}^{-3}e/{\mathrm{a}.\mathrm{u}.}^3$) of the majority-spin $t_2$ states associated with a neutral Ga vacancy in a 64-atom GaN supercell, showing the localized nature of defect states. About 60% of the $t_2$ state charge is contained within the isosurface.

Figure 3

Isosurface charge density plot (${\rho }^0=7.8\times {10}^{-4}e/{\mathrm{a}.\mathrm{u}.}^3$) of the majority-spin $t_2$ states associated with a neutral Ga vacancy in a 216-atom GaN supercell, showing both the localized nature and the extended tails of the defect wave functions.

Figure 4

Schematic of the exchange mechanism. For neutral cation vacancies, virtual hopping results in an AFM ground state; the same mechanism leads to a FM coupling between charged vacancies.