  We report on the appearance of magnetic stripes in MnAs/GaAs(100) epilayers
at temperatures well below the ferromagnetic transition of the system. The
study has been performed by ferromagnetic resonance experiments (FMR) on MnAs
epilayers grown on (100) and (111) GaAs substrates. The FMR spectra of the
MnAs/GaAs(100) samples at 180 K reveal the appearance of zones of different
magnetic behavior with respect to the low-temperature homogeneous ferromagnetic
phase. The angular and the temperature dependence of the spectra serve us to
detect the inter-growth of the non-magnetic phase into the ferromagnetic phase
at a very early stage of the process. The experimental data show that the new
phase nucleates in a self-arranged array of stripes in MnAs/GaAs(100) thin
films while it grows randomly in the same films grown on GaAs(111).
