The properties of isolated Mn substituted at Ga sites, (MnGa), were investigated. It was questioned here whether alternative lattice sites were favored and what the magnetic effects might be. Under the As-rich (Ga-poor) conditions which prevailed during growth, it was found that the formation energies were lower for MnGa than for interstitial Mn (Mni). As the Fermi energy shifted towards the valence band maximum via external p doping, the formation energy of Mni was reduced relative to MnGa. Under epitaxial growth conditions, the solubility of both substitutional and interstitial Mn were strongly enhanced over that possible under bulk growth conditions. The high Mn concentration which was attained during the epitaxial growth of p-type material offered the possibility of Mn atoms forming small clusters. Various types of cluster were considered: including Coulomb-stabilized clusters involving two MnGa and one Mni. It was found that, while isolated Mni were donors (and therefore destroyed ferromagnetism), complexes such as (MnGa-Mni-MnGa) were more stable than complexes involving MnGa-MnGa-MnGa. The former complexes exhibited a partial or total quenching of holes, but the Mni of the complexes provided a channel for ferromagnetic arrangement of the spins on the two MnGa within the complex. It was suggested that ferromagnetism in Mn-doped GaAs arose from holes due to isolated MnGa as well as from strongly Coulomb-stabilized MnGa-Mni-MnGa clusters.

Ferromagnetism in Mn-Doped GaAs due to Substitutional-Interstitial Complexes. P.Mahadevan, A.Zunger: Physical Review B, 2003, 68[7], 075202 (8pp)