We attempt to address here an actual question facing material scientists today, namely novel materials for devices explicitly utilizing the electron spin for microelectronic applications. Among many, GaMnAs is commonly considered as a promising material for such applications. Most of the studies on this compound were devoted to the uniform ternary alloy which is a diluted ferromagnetic semiconductor; however it is fairly easy to obtain GaMnAs samples containing precipitations of ferromagnetic MnAs. These precipitations were usually considered as a major drawback. However, since MnAs is a metallic ferromagnet with Curie temperature of about 318 K, it is possible to prepare the GaAs:MnAs system in such a way that small ferromagnetic nanoparticles are immersed in the semiconductor host lattice. Such a composite material could be considered as a good semiconductor filled with nanomagnets providing a built-in magnetic field at room temperature. These magnetic MnAs nano-clusters can be produced from a single-phase GaMnAs material by the post-growth annealing at temperatures higher than 500 C. We focus here on the atomic and electronic structure of the Mn atoms relating to the cluster formation. The changes in the electronic structure of Mn, Ga and As atoms in (Ga,Mn)As layers after high temperature annealing were determined by x-ray absorption near edge spectroscopy (XANES). The experimental spectra were compared with the predictions of the ab initio full multiple scattering theory (FEFF 8.4 code). The nominal concentration of the Mn atoms in the investigated samples was 6%, therefore we do not observe changes in the electronic structure of Ga and As. However, we found considerable changes in the case of Mn atoms.
This work was partially supported by national grant of Ministry of Science and High Education N202-052-32/1189 as well as by DESY, MAX-lab and the European Community under Contract RII3-CT-2004-506008 (IA-SFS).

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