Dependence of magnetic circular dichroism on doping and temperature in In1-x Mnx As epitaxial films

To elucidate the nature of the exchange interactions in ferromagnetic In1-x Mnx As, the composition and temperature dependencies of the magnetic circular dichroism (MCD) spectrum were investigated. The spectrum is comprised of both a broad transition from 1.6 to 3.0 eV and a negative peak between 2.68 and 2.8 eV associated with L -point sp-d transitions. The spectral response of the MCD is strongly temperature dependent. The magnitude of the negative peak and magnetization increases significantly with decreasing temperature for T<150 K due to an increase in the sp-d exchange coupling. In contrast, the broad background remains relatively constant over the same temperature range. The difference in the temperature dependence of the two negative background and peak corroborates previous assertions that the two features are due to two distinct transitions. For a series of films ranging from x=0.020 to 0.063, the magnitude of the MCD peak and background exhibits a superlinear dependence on Mn concentration at room temperature. The superlinear dependence is attributed to the formation of atomic scale, tetrahedrally coordinated MnAs clusters.