Structural and magnetic properties of MnAs nanoclusters formed by Mn ion implantation in GaAs

Ferromagnetic (FM) nanostructures embedded in semiconductors are of fundamental interest since their physical properties could be used in new devices such as memories, sensors or spintronics. In this work, we present results obtained on the synthesis and characterization of nanosized MnAs ferromagnets buried in GaAs. These nanocrystals are formed either by single Mn implantation or Mn + As co-implantation at room temperature into GaAs wafers at 141 and 180 keV respectively. Two doses, 1 x 10{sup 16} and 2 x 10{sup 16} ions {center_dot} cm{sup -2} for each impurity, are tested. Pieces of the wafers are then annealed by RTA or classical furnace annealing at various temperatures under N{sub 2} atmosphere for increasing times. HRTEM and diffraction analysis show that under such conditions MnAs precipitates form with a regular hexagonal structure, the 3m orientation-relationship of precipitates with respect to the matrix offers the most energetically stable configuration. Size distributions are systematically extracted from statistical analysis of ''2 beam'' TEM images. The precipitate mean diameters of nanocrystals populations range from 9 to 13 nm depending on the annealing conditions. Magnetization measurements by SQUID magnetometry on the same samples reveal a progressive transition from a superparamagnetic behavior at room temperature to an FM one at 2K, reflecting a distribution of blocking temperature, due to distribution of size and to dipolar interactions. Curie temperatures in the range of 360K were measured.