Mn-impurity-conduction-mediated ferromagnetic ordering in Ga1−xMnxAs (x = 0.034 and x = 0.050)

The conduction mechanism expected for a diluted magnetic semiconductor, Ga1−xMnxAs (x = 0.034 and x = 0.050), is presented. Infrared (IR) and far-infrared (FIR) spectroscopy reveal that the free carrier absorption spectrum cannot be observed, and that the broad absorption peak observed at 2000 cm−1 is ascribable to the optical transition from the ground states of the isolated acceptor, acceptor pair, triplet and cluster to their excited states and to the valence bands. The absorption peak exhibits Stokes shift from 2100 to 1900 cm−1 with decreasing temperature below the ferromagnetic ordering temperature TC = 43 K. These phenomena indicate that holes are redistributed in the spin splitting states leading to magnetization. Since Mn acceptors are in a random system, holes are weakly localized at Mn-sites and hop in the acceptor complexes. The absorption coefficient integrated from 50 to 150 cm−1 increases with decreasing temperature from TC and with increasing external magnetic field. These phenomena have been explained in terms of correlated many-hole hopping (CMHH). That is, the absorption is proportional to the number of holes which simultaneously hop in the Mn acceptor states. CMHH conduction depends mainly on the exchange energy between the hole spins and localized Mn spins. In addition, the many-body effects of CMHH conduction have been confirmed by IR and FIR reflection spectroscopy and analysis of the spectra. The effective mass ratio enhancement has been found to be as large as 1.7 at 10 K below TC. Thus, the authors have concluded that CMHH conduction mediates the ferromagnetic ordering in Ga1−xMnxAs (x = 0.034 and x = 0.050).