Neutral and charged embedded clusters of Mn in doped GaN from first principles

Abstract

Based on extensive density-functional theory calculations, the spatial distribution and magnetic coupling of Mn atoms in Mn:GaN have been reinvestigated by doping up to five Mn atoms in large supercells, where the formation energies and the electronic structure for both the neutral and charged valence states are studied. The doped Mn atoms have a strong tendency to form substitutional Mn-N-Mn bonded embedded clusters with short-range magnetic interactions, where the long-range wurtzite structure is maintained. While for neutral pair doping the coupling is ferromagnetic regardless of the distance and orientation of the Mn atoms, the negatively charged states tend to weaken the parallel coupling. Significantly, for larger ͑than pair͒ cluster configurations for both neutral and all the energetically favorable charged states, states containing antiparallel coupling are always favored. Thus, we argue that the "giant cluster moment" in Mn:GaN, as proposed by Rao and Jena based on study of free clusters ͓Phys. Rev. Lett. 89, 185504 ͑2002͔͒ and calculated by Sandratskii et al. ͓Phys. Rev. B 71, 045210 ͑2005͔͒, is not applicable for "larger" ͑Ͼ2͒ Mn-cluster-doped GaN. The size of the supercells employed and the atomic relaxation are found crucial for an accurate description, and are partially responsible for these discrepancies. Also important is that the electrical conductivity of Mn:GaN depends sensitively on the valence states, where the negatively charged state Mn 2+ ͑d 5 ͒ exhibits highly insulating character as observed in experiments. The formation of embedded clustering leads to a strong local structural distortion and a significant spin polarization on neighboring N atoms due to hybridization of Mn 3d and N 2p orbitals. Our results highlight the intrinsically complex nature of the spatial distribution and magnetism in transition metal doped dilute magnetic semiconductors, and can rationalize some hitherto puzzling experimental observations, notably the low saturation moments, the contracted lattice constants, and the highly insulating behavior.

Nearest-neighbor Mn antiferromagnetic exchange inGa1−xMnxN

“…51 It is also interesting to note that a recent calculation finds that Mn ions within groups of two or more couple antiferromagnetically for all but the highest Mn oxidation states. 52 Our analysis supports the results of Ref. 14 that show that the most structurally perfect, well-characterized films of Ga 1−x Mn x N result in ferromagnetism at low temperatures ͑ ϳ10 K͒ only.…”

Antiferromagnetism with spin polarization of GaN-based diluted magnetic semiconductors

“…However, other effects such as second neighbors, number of neighbors, and direction of interactions are responsible for some deviations from this behavior, specially in the case of MnCoGaN where strong magnetic couplings are obtained even for an intermediate value of the distance between TM atoms in the cell. The results for Cr-Cr and Mn-Mn magnetic interactions can be compared with previous results obtained by Cui et al 28,29 In the mentioned papers, there are results using the same cell that we used for Cr and Mn doping of GaN: ͑i͒ the results for Mn doping in GaN ͑Ref. 29͒ are in good agreement with us; the ground state is FM in all configurations and in general way, the energy difference between FM and AFM states decrease strongly with the distance; and ͑ii͒ the results for Cr doping, although also show a very fast decay of the total-energy difference between FM and AFM states, 28 similar with our result, there is a oscillatory behavior in the ground state of the configurations between FM and AFM states.…”

What is the Valence of Mn inGa1−xMnxN?

“…This is because the wave nature of the GaN orbitals, after being integrated out, generates effective magnetic couplings that are impurity-configuration dependent between the Mn WOs at different sites. For instance, the magnetic coupling does not only depend on the distance between pairs of Mn impurities [24], but also on the position of other nearby Mn impurities, corresponding to three-body and four-body interactions [64].…”