Synthesis of MnAs ferromagnets by Mn+ ion implantation

Ferromagnetic (FM) nanostructures embedded in semiconductors are attracting interest because their physical properties could be used in new devices such as memories, sensors or more generally involving "spintronics". In this work, we will present experimental results on the influence of the thermal annealing on the structural and magnetic properties of nanosized MnAs ferromagnets buried in GaAs. These nanocrystals have been obtained by Mn+As co-implantation at a, dose of 2 x 10(16) ions cm(-2) for each species, performed at room temperature into GaAs wafers followed by thermal annealing at 750degreesC. Increasing the duration of the annealing process (t(anneal) = 15,60, and 120 s) leads to a reduction of the Mn atomic densities. High-resolution transmission electron microscopy and diffraction analysis exhibit unambiguously a population of MnAs precipitates located at the projected range. Their mean diameters are ranged from 8 to 10 nm as t(anneal) is longer. The nanocrystals display the expected ferromagnetic behavior with Curie temperatures in the range of 360 K. The extracted spontaneous magnetizations from the FM response are found slightly smaller than expected for the bulk phase, probably due to a small fraction of diluted Mn. The influence of the nanoparticle size histogram and concentration on the superparamagnetic behavior is briefly discussed. (C) 2002 Published by Elsevier Science B.V.