Mn disorder effects play an important role in ferromagnetism of III-Mn-V's. In ferromagnetic GaMnAs, a structurally metastable material, Manganese is introduced with compositions above 1 at.%, a much higher level than the equilibrium doping limit. Consequently, an atomic ordering of Mn atoms in the lattice (of short range order type) is expected to exist (even before any Mn-enriched precipitates occur). It is not clear to what extent it is present in ferromagnetic layers [Ref. 1]. To study Mn-Mn short range order we intended to prepare Ga(1-x)Mn(x)As crystals with Mn composition only slightly above the equilibrium doping limit. Using annealing techniques, an atomic order could, in principle, be controlled and studied.
Bulk Ga(1-x)Mn(x)As crystals with Mn composition up to 0.1 at.% were grown using equilibrium technique (Czochralski). Samples were characterized by magnetization measurements (SQUID) and by Hall effect. GaMnAs samples with Mn composition up to 0.1 at.% were paramagnetic, Mn composition was evaluated fitting Brillouin function. Sample with 0.1 at.% Mn was studied by EXAFS technique (at HASYLAB, Hamburg) to evaluate local environment of Mn ions in the lattice.
GaMnAs samples were subjected to an annealing at various temperatures (700-1200 deg C) during 0.5 to 100 hours. Such thermal treatment was expected to change properties of GaMnAs if Mn composition exceeded an equilibrium value. Hole concentration was used as a most sensitive probe of changes caused by annealing. For GaMnAs with 0.1 at.% of Mn no changes of electrical properties were observed at any annealing temperature. This indicates that Mn composition of 0.1 at.% is still below an equilibrium doping limit.
Such a conclusion is confirmed by EXAFS studies, which revealed a well-resolved structure up to 4-th coordination shell, which could be fitted assuming solely substitutional Mn(Ga) incorporation.

[1] T. Slupinski, H. Munekata, A. Oiwa, APL 80 (2002) 1592

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