Abstract :
We present our results obtained for Mn-doped GaAs quantum wells where the evidence of the ferromagnetic transition at relatively high temperatures was found at unusually small Mn concentrations. Surprisingly, the evidence disappeared with an increase of the dopant concentration. Both observed values of resistance at small temperatures and its temperature dependencies evidenced that the samples are deep in the insulating regime. Thus the values of the overlapping integrals for hopping can hardly explain the large values of Curie temperatures T c ≃ 100 K . We develop a theoretical model qualitatively explaining the experimental results based on the concept of virtual Anderson transition.
