Nitrogen vacancy and ferromagnetism in Cr-doped GaN: First-principles calculations

Nitrogen defects and their effect on the ferromagnetism (FM) in Cr-doped GaN have been systematically investigated by first-principles. Four considered configurations including one N vacancy (VN), single substitutional Cr, double substitutional Cr, and complex of Cr–VN are all ferromagnetic. The lowest energy arrangements for double Cr-doped (or Cr–VN) GaN occur at the nearest Cr–Cr (or Cr–VN) distance. One Cr contributes 84.3% of the total magnetic moment (2.533 μB), while one Cr–Cr pair leads to 5.998 μB moment (more than twice moment of one Cr) by the strong d–d spin coupling, which is mediated by two Cr 3d states antiferromagnetically coupling with the “commonly shared” N 2p states, and driven by a double exchange mechanism. The VN can enhance the FM by adding about 1 μB to the Cr moment but reduce the FM spin exchange interaction between the nearest Cr–Cr pairs, so experimentally, high-performance samples may be synthesized by controlling N pressure.