Blueshift of magneto-optical spectra and ferromagnetic ordering in (GaMn)As/AlAs ultrathin (<5 nm) heterostructures

We have studied transmission magneto-optical spectra of (GaMn)As/AlAs ultrathin heterostructures. In the heterostructures whose (GaMn)As thickness d is thinner than 5 nm, the magneto-optical spectra are significantly blueshifted with the decrease of d, whereas the heterostructures with thicker (GaMn)As (>12 nm) show magneto-optical spectra similar to that of bulk (GaMn)As, and the mechanisms of the blueshift are discussed. Furthermore, we have observed ferromagnetic ordering even in the ultrathin (GaMn)As (d=2 and 5 nm) samples with their transition temperature of 70 K.