We report the formation of nanosize ferromagnetic MnAs crystallites imbedded in low-temperature grown GaAs usingMn+ion implantation and subsequent annealing. The structural and magnetic properties of the crystallites have been characterized by transmission electron microscopy, electron beam induced x-ray fluorescence, and superconducting quantum interference device magnetometry. After an optimized thermal annealing at 750 °C, MnAs crystallites of 50 nm in size are formed. These nanomagnets show room temperature ferromagnetism. ©1997 American Institute of Physics.