MnAs films were grown by MBE on GaAs(0 0 1) and GaAs(1 1 3)A. We performed a systematic study about the correlation between their structural and magnetic order, which is intimately connected with the phase transition between beta-MnAs and alpha-MnAs during sample cooling after growth. A phase separation process leads to a self-organized structure on the sub-micron scale with alternating ferromagnetic and nonferromagnetic stripes. The well-ordered structure contains defects which correspond to typical crystal imperfections on the atomic level. (C) 2002 Elsevier Science B.V. All rights reserved.