Spin polarization dependent far infrared absorption in Ga1-xMnxAs

Infrared and far infrared transmission spectra have been measured for the ferromagnetic Ga1-xMnxAs (x = 0.034 and x = 0.050) between 10 and 12000 cm(-1). A broad peak is observed around 1600 cm(-1), and this energy is close to the deep acceptor level of Mn impurity in GaAs. A finite featureless absorption, spreading between this peak and the band gap, due to inter valence band transitions is also observed. At the same time, a non-zero absorption is observed in the lowest frequency studied, which points to the absorption by mobile holes related to the metallic DC conductivity. The frequency dependence in this range, which differs markedly from the Drude model, demonstrates a strong influence of electrostatic and magnetic disorder on the optical conductivity. In particular, the existence of a strong coupling between the Mn spins and the holes explains the considerable increase of the far infrared absorption below the ferromagnetic ordering temperature.