Spherical MnAs ferromagnetic particles with controllable diameters (5-30 nm) are embedded in a high quality GaAs matrix. The particles are formed in a two step process consisting of the epitaxy of a homogeneous Ga1-xMnxAs layer at low temperatures using molecular beam epitaxy followed by phase separation upon annealing. During the annealing step, the excess arsenic in the as-grown film forms magnetic MnAs precipitates with the Mn from the Ga1-xMnxAs lattice. Structural and room-temperature magnetic properties of the heterogeneous GaAs:MnAs films are described. The magnetic MnAs rich layers can be incorporated into semiconductor heterostructures as demonstrated by growing (GaAs/AlAs) multiple quantum well structures in combination with GaAs:MnAs layers. (C) 1996 American Institute of Physics.