Intrinsic hole localization mechanism in magnetic semi- conductors

– 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 (Ga1−x,Mnx)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