We present a unified interpretation of experimentally observed magnetic circular dichroism (MCD) in the ferromagnetic semiconductor (Ga,Mn)As, based on theoretical arguments, which demonstrates that MCD in this material arises primarily from a difference in the density of spin-up and spin-down states in the valence band brought about by the presence of the Mn impurity band, rather than being primarily due to the Zeeman splitting of electronic states.

We have studied the dependence of the cubic and uniaxial magnetic anisotropy terms in Ga 1−x Mn x As on temperature T and hole concentration p. For the purpose of this study we prepared a series of Ga 1−x Mn x As layers with low Mn concentration ͑x Ϸ 0.01͒, codoped by Be in the range 3.0ϫ 10 19 Ͻ p Ͻ 8.5ϫ 10 19 cm −3 , and grown on hybrid ZnSe/ GaAs substrates. The use of such hybrid substrates was intended to obtain Ga 1−x Mn x As layers in which-due to the small lattice mismatch between Ga 0.99 Mn 0.01 As and ZnSe-the uniaxial and the cubic anisotropy terms are comparable, so that the contributions of both types of anisotropy could be investigated. The effects of magnetic anisotropy were studied by polar magneto-optical Kerr effect, which allowed us to monitor the reversal process of perpendicular magnetization. The results showed that cubic anisotropy is highly sensitive to both p and T. Specifically, we have found that in samples with high p the cubic anisotropy term is dominant at low T, but decreases rapidly as T increases. In sharp contrast, uniaxial anisotropy shows only a weak dependence on p and T, thus dominating at temperatures close to T C even in samples with high p. These results show that magnetic anisotropy and the magnetization reversal process in Ga 1−x Mn x As can be engineered by an appropriate choice of the temperature and carrier concentration.

We report magnetic circular dichroism measurements on thin GaMnAs films with low Mn concentration, which reveal that exchange-induced spin splitting of the band edges occurs only in samples that show ferromagnetic order, and is not observed in paramagnetic samples. These results lead to the conclusion that Mn ions in the A0 configuration (d5+hole) provide the only mechanism for exchange interaction between Mn spins and band carriers. We also show that there is a linear relation between the observed exchange-induced splitting of the band edges and the Curie temperature, pointing to a common origin of the band edge splitting and ferromagnetism in GaMnAs.

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