Current interest in spin-based electronics has generated a demand for materials in which magnetic properties occur simultaneously with a strong spin-dependent response of charge carriers. In this connection III-Mn-V ferromagnetic semiconductors – and GaMnAs in particular – continue to attract attention. Although there is general consensus that the ferromagnetic coupling between Mn spins in GaMnAs is mediated by holes contributed by Mn ions, the nature of the hole wavefunctions as well as the their location (valence band vs. an impurity band located at about 110 meV above the top of the valence band) is one of the most controversial topics in this field. There are many theoretical models discussed in the literature, but as yet there is no conclusive experimental result that can definitively resolve this issue. In order to address this controversy, we studied MCD on a series of Ga<SUB>1-x</SUB>Mn<SUB>x</SUB>As layers grown by MBE, with x ranging from 0.02 to 0.06. As was shown by Berciu et al , the MCD signal in GaMnAs arises primarily from a difference in the density of spin-up and spin-down states in the valence band brought about by the presence of the Mn impurity band. In particular, MCD spectra, for an as-grown and an annealed sample with x = 0.06, show the same general features as those observed on samples with Mn concentration ~0.01 reported in . Specifically, the signal rises sharply in the vicinity of the energy gap and forms a very broad spectrum with two (as grown) or three (annealed sample) distinct peaks. The peaks correspond to the maximum contribution to the IB from the heavy hole (lower energy peak) and light hole (higher energy peak) bands. In the annealed sample the contribution from the Γ7 band is also clearly visible.
In contrast, the spectra taken on Ga<SUB>0.98</SUB>Mn<SUB>0.02</SUB>As samples co-doped with Be reveal that the MCD signal disappears in the vicinity of the energy gap for samples with Be concentration higher than 1×10<SUP>20</SUP> cm<SUP>-3</SUP>. In the case of Be-doped samples the Fermi level lies in the valence band, and consequently interband transitions at the band gap disappear. By contrast, the strong MCD signal observed at the band gap in undoped GaMnAs samples indicates a difference in the density of spin-up and spin-down states at the top of the valence band, and consequently points to the fact that the Fermi level must lie in the impurity band.
 M. Berciu et al., Phys. Rev. Lett. 102, 247202 (2009).