Annealing effects on the structural and magnetic properties of Fe-Al silica nanocomposites prepared by sequential ion implantation

The nanostructural and magnetic properties of FeAl-SiO(2) granular solids prepared by sequential ion implantation have been investigated as a function of the annealing atmosphere (either oxidizing or reducing) and implantation order. Nanoscopic particles with a bcc structure were found in both as-implanted samples. In the sample Al-Fe prepared by implanting first the Al ions and later the Fe ions, the lattice parameter indicates the presence of practically pure iron nanoparticles. On the other hand, in sample Fe-Al with the implantation order inverted, the lattice parameter is consistent with the presence of an iron rich iron-silicon alloy. The magnetic data confirm the presence of the pure Fe and the Fe-Si alloy in the as-implanted samples and the absence of FeAl intermetallic compounds. The annealing in Ar/H(2) promotes the growth of the clusters and increases the Si content in the particles in both samples. In Fe-Al sample, this induces a disorder-order phase transition from the disordered Fe-Si solid solution to the Fe(3)Si phase and the coprecipitation of the ordered FeSi phase. The magnetic moment increases after the annealing in Ar/H(2) due to the incorporation of the iron atoms dispersed in the matrix and to the higher crystalline order. The annealing in air is responsible essentially of the growth of the Fe-Si clusters in both samples. On the other hand, in sample Al-Fe the oxygen interacts with the pure iron clusters by promoting the Fe(2)O(3) formation.