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We report on formation of nanowires during molecular beam epitaxial growth of GaMnAs ferromagnetic semiconductor. The development of nanowires occurs when GaMnAs is grown at conditions leading to phase separation, i.e. formation of MnAs nano-clusters at the GaMnAs surface. When the MBE growth is continued with the presence of MnAs clusters, the growth mode changes from two dimensional layer-by-layer to three-dimensional growth at which nanowires are formed. Most of the nanowires grow along <111> directions independently of the substrate orientation – both 100 and 111B GaAs substrates were tested. Nanowires were grown at temperatures between 250 and 400 °C. Low growth temperatures (300 °C and below) resulted in strongly tapered nanowires with typical diameters of about 50 nm at the base and 5 nm at the tip for lengths of about 700 nm. For higher growth temperatures the tapering is less pronounced. Individual nanowires investigated by energy-dispersive X-ray analysis in transmission electron microscope appear as Mn-rich at their ends and at the base. The growth of nanowires is catalyzed by MnAs clusters, which were identified at nanowire tips, although some of the nanowires do not show Mn-rich tips. In contrast to the well-known vapor-liquid-solid growth mechanism of nanowires, the catalyzing MnAs clusters are not liquid, so another mechanism must be responsible in the present case. Nanowires grown on GaAs(100) substrates are inclined to the surface normal by about 55 degrees, due to their growth along <111> direction. Typically the outer side of the nanowires (i.e. that facing the evaporation sources) exhibit branches. It is likely that the branches are formed due to the appearance of secondary MnAs clusters formed during the growth. The nanowires grown on GaAs(111)B substrates are perpendicular to the substrate and do not have branches. The samples exhibit ferromagnetic properties up to room temperature. However, both the high Curie temperature and high coercive field values (up to 500 Oe) indicate that the magnetic properties are dominated by the MnAs clusters rather than by diluted GaMnAs phase. |