Anomalous Fermi level behavior in GaMnAs at the onset of ferromagnetism

We present the systematic study of the resonant tunneling spectroscopy on a series of ferromagnetic-semiconductor Ga1-xMnxAs with the Mn content x from ~0.01 to 3.2%. The Fermi level of Ga1-xMnxAs exists in the band gap in the whole x region. The Fermi level is closest to the valence band (VB) at x=1.0% corresponding to the onset of ferromagnetism near the metal-insulator transition (MIT), but it moves away from the VB as x increasing or decreasing from 1.0%. This anomalous behavior of the Fermi level indicates that the ferromagnetism and MIT emerge in the Mn-derived impurity band.