We have successfully grown single-crystal MnAs/AlAs/MnAs magnetic tunnel junctions (MTJs) having extremely smooth interface morphology on exact (1 1 1)B GaAs substrates, where a several-monolayer-thick ultrathin GaAs buffer layer was used to avoid the unwanted formation of pyramidal facets that would otherwise be spontaneously evolved on a thick GaAs buffer layer surface. The surface morphology of the MnAs film grown on an ultrathin GaAs buffer layer is strongly dependent not only on the growth temperature of the MnAs film but also on the thickness of the ultrathin GaAs buffer layer, and became completely flat by optimizing them. When MnAs/AlAs/MnAs MTJs with flat interfaces were epitaxially grown on an ultrathin GaAs buffer layer, they exhibited clear double-step features in magnetization loops and tunneling magnetoresistance. On the other hand, magnetic properties of the MTJs grown on a thick GaAs buffer layer were severely deteriorated by interface-roughness-induced interlayer coupling. (C) 2002 Elsevier Science B.V. All rights reserved.