Growth of Mn on GaAs(100) is studied by photoemission and electron-ene
rgy-loss spectroscopy. We find that the first layer of Mn grows on GaA
s(100) in a two-dimensional mode, where about 10% of the Mn atoms take
part in the chemical reaction with the substrate. The electronic stru
cture of the ultrathin film of Mn is found to be very different from t
hat of bulk alpha-Mn, and the possibility that magnetic ordering exist
s in the ultrathin overlayer of Mn is proposed.