  X-ray diffraction, transmission electron microscopy, and magnetization
measurements are employed to study the structural and magnetic properties of
Mn-rich (Mn,Ga)As nanocrystals embedded in GaAs. These nanocomposites are
obtained by moderate (400C) and high temperature (560 and 630C) annealing of
(Ga,Mn)As layers with Mn concentrations between 0.1 and 2%, grown by molecular
beam epitaxy at 270oC. Decomposition of (Ga,Mn)As is already observed at the
lowest annealing temperature of 400C for layers with initial Mn content of 1%
and 2%. Both cubic and hexagonal (Mn,Ga)As nanocrystals, with similar diameters
of 7 - 10 nm are observed to coexist in layers with an initial Mn content of
0.5% after higher temperature annealing. Measurements of magnetization
relaxation in the time span 0.1 - 10 000 s provide evidence for
superparamagnetic properties of the (Mn,Ga)As nanocrystals, as well as for the
absence of spin-glass dynamics. These findings point to weak coupling between
nanocrystals even in layers with the highest nanocrystal density.
