Below band gap Faraday and Kerr measurements in ferromagnetic GaMnAs

We have studied the Faraday and Kerr effects in a series of ferromagnetic GaMnAs films in the 115-1500 meV energy range. This provides a direct magneto-optical probe of the valence band, which is critical to understanding the ferromagnetic origin in this material. Previous magneto-optical studies probed the region around the band gap (1.5 eV) where numerous contributions to the optical response were inferred: ferromagnetic, paramagnetic, valence band, impurity bands, intra-d level or intra-gap defect levels. We compare our experimental results with predictions from mean field Zener model calculations. We find that the results are consistent with a picture in which the Fermi level resides inside the spin split valence band. Many-body band renormalization effects have to be included in order to model the results quantitatively.