  The high-spectral-resolution spectroscopic studies of the energy gap
evolution, supplemented with electronic, magnetic and structural
characterization, show that the modification of the GaAs valence band caused by
Mn incorporation occurs already for a very low Mn content, much lower than that
required to support ferromagnetic spin - spin coupling in (Ga,Mn)As. Only for
n-type (Ga,Mn)As with the Mn content below about 0.3% the Mn-related extended
states are visible as a feature detached from the valence-band edge and partly
occupied with electrons. The combined magnetic and low-temperature
photoreflectance studies presented here indicate that the paramagnetic -
ferromagnetic transformation in p-type (Ga,Mn)As takes place without imposing
changes of the unitary character of the valence band with the Fermi level
located therein. The whole process is rooted in the nanoscale fluctuations of
the local (hole) density of states and the formation of a
superparamagnetic-like state. The Fermi level in (Ga,Mn)As is coarsened by the
carrier concentration of the itinerant valence band holes and further
fine-tuned by the many-body interactions.
