The thickness dependence of the magnetic properties of MnAs films grown on GaAs(001) and their alteration by post-growth annealing has been studied. It is shown that a phase separation into α- and α-MnAs forming ferromagnetic and paramagnetic stripes, respectively, and the magnetic interaction between these stripes determine the properties of the films. Using a specific saturation magnetization MS* which refers to the actual volume of α-MnAs, the thickness dependence of Ms* can be described in an universal way. Values well above -1100 emu/cm3 obtained for the optimum film thickness are considered as an intrinsic property of nearly defect-free-MnAs in the fully magnetized state. © 2005 IOP Publishing Ltd.