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

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 present results on the synthesis and the characterization of nanosized MnAs ferromagnets buried in GaAs. These nanocrystals can be formed either by single Mn implantation or by Mn +As co-implantation at room temperature into GaAs wafers followed by thermal annealing. High-resolution transmission electron microscopy and diffraction analysis show that the MnAs precipitates exhibit the regular hexagonal structure with a 3m orientation relationship with respect to the matrix. The mean diameters of the nanocrystals range from 9 to 13 nm depending on the annealing conditions. Magnetization measurements by superconducting quantum interference device magnetometry reveal an FM state. Upon warming, a progressive transition to a superparamagnetic state is evidenced, probably due to the distribution of individual blocking temperatures, resulting both from the size distribution of the nanocrystals and from dipolar interactions. Curie temperatures in the range of 360 K are measured, significantly higher than for bulk MnAs. (C) 2003 Elsevier Science B.V. All rights reserved.