Intrinsic hole localization mechanism in magnetic semiconductors

Abstract

Abstract. -The interplay between clustering and exchange coupling in magnetic semiconductors for the prototype (Ga1−x, Mnx)As with manganese concentrations x of 1/16 and 1/32 in the interesting experimental range is investigated. For x ∼ 6%, when all possible arrangements of two atoms within a large supercell are considered, the clustering of Mn atoms at nearest-neighbour Ga sites is energetically preferred. As shown by spin density analysis, this minimum energy configuration localizes further one hole and reduces the effective charge carrier concentration. Also the exchange coupling constant increases to a value corresponding to lower Mn concentrations with decreasing inter Mn distance.Including spin information into semiconductor electronics has enormous potential for new applications (see Ref.[1] for a review on magnetoelectronics). An interesting situation arises when the carriers responsible for metallic behaviour are of the same spin, in the so called half-metals [2]. Within this class of materials, the magnetic semiconductors are particularly challenging, not only because they can replace magnetic elements in storage media, but because they can be used as spin injectors into normal semiconductors [3,4] for completely novel applications. In the field of magnetic semiconductors a new chapter has been opened by the III-V diluted magnetic semiconductors (DMS), especially with (Ga, Mn)As.Experimentally (Ga, Mn)As can be manufactured using low-temperature molecular beam epitaxy (LT-MBE) [5,6,7,8] and its magnetic properties have been measured with numerous techniques [5,6,7,8,9,10,11]. (Ga,Mn)As is ferromagnetic (see e.g. Refs. [6, 7, 8]), with a Curie temperature as large as 110 K in the Mn concentration range between 5% and 10%. In addition, the temporal evolution of the magnetization during annealing shows that first ferromagnetism is enhanced and then reduced, [11], suggestive of clustering processes.In this Letter, we first survey the current state of theoretical research in the field. To overcome all the possible sources of errors a new benchmark is needed. We then present calculations for magnetic semiconductors, studying (Ga 1−x , Mn x )As in different magnetic configurations. In particular, we assess the role of Mn clustering in the lattice (see Fig.1 describing the considered cases). Finally, the clustering results drive us to reexamine the findings of previous calculations that address the role of the exchange coupling parameter within mean-field theories.The ferromagnetism (FM) in (Ga,Mn)As is mediated by holes that are antiferromagnetically (AFM) coupled to the Mn. Theoretical studies using state-of-the-art computational c EDP Sciences

Diffusion and clustering of substitutional Mn in (Ga,Mn)As

“…We also show that this diffusion leads to Mn clustering, which reduces T C [10,11]. Although the formation of Mn Ga clusters has been shown to be energetically * Electronic address: hra@fyslab.hut.fi favorable [11,13,14,15], it requires an abundance of mobile gallium vacancies. The recent discovery of rather high gallium vacancy (V Ga ) concentrations in (Ga,Mn)As up to 10 18 cm −3 [16] gives us good reason to consider this mechanism plausible, but the mobilities of V Ga and Mn Ga in (Ga,Mn)As are unknown.…”

“…The binding energies are calculated as the difference in total energy of the pairs located at nearest-neighbor and fifth-nearest-neighbor separations on the Ga fcc sublattice in 64 atom supercells that are fully relaxed, as described in Ref. [15]. In the 107 atom supercell we use a 4 × 4 × 3 k-point sampling mesh giving a similar k-point density as that used in Ref.…”

“…In the 107 atom supercell we use a 4 × 4 × 3 k-point sampling mesh giving a similar k-point density as that used in Ref. [15].…”

“…The Curie temperature T C depends mainly on the concentration of Mn clusters [cl] [defined as the proportion of the number of clusters (including monomers) to the number of Ga sites] [11,15], i.e. T C remains approximately unchanged at fixed [cl] even when [Mn] is varied.…”

The quest for dilute ferromagnetism in semiconductors: Guides and misguides by theory

“…Statistics tells us that the probability of two or more defects on one of the sublattices in a zinc blende, wurtzite, or rock salt crystal occupying neighboring sites is > 50% for defect concentrations > 5% (∼ 10 20 /cm 3 ) [69]. Once a defect pair has another defect next to it (e.g., with a 50% probability), it has a lowered point group symmetry and consequently a different electronic spectrum, and is likely to exhibit completely different magnetic interactions compared to its isolated relatives [48,70]. This has the detrimental effect that magnetic interactions no longer can be mapped on Dirac-van Vleck-Heisenberg type models (E = ΣJ ij s i · s j ) [71].…”