Investigation of the creation of Fe nanoparticles in MgO by ion beam synthesis

Recently, the investigation of metallic nanoparticles embedded in dielectric oxides has become a topic in magnetic data storage development. Especially the creation of high density data storage devices due to ion beam synthesis of magnetically, highly anisotropic FePt nanoparticles in sapphire yielded promising results. On the other hand, pure Fe-nanoparticles have been created in sapphire, Y-stabilized ZrO2 or MgO. The motivation for the latter includes the search for magneto-optically active surfaces as well as pronounced magnetoresistance.
For doping MgO single crystals with Fe, ion implantation is a promising technique. First extensive studies were performed by Perez et al. who investigated the annealing behaviour of samples implanted with different doses of 57Fe at different energies with conversion electron Mössbauer spectroscopy (CEMS) [1]. They obtained up to 20 % of the implanted iron atoms in a-Fe precipitates while the remaining Fe-atoms were found to be in different Fe2+ and Fe3+ charge states.

In this paper, we propose a method for the creation of large fractions of a-Fe nanoparticles (up to 60%). MgO(001) single crystals were implanted with of 6x1016 cm-2 ions of the Mössbauer isotope 57Fe at 100 keV. The implantation temperature was varied from room temperature up to 800 °C. Moreover, post annealing was investigated. Samples were characterized by CEMS, RBS, TEM and XRD. We found that the temperature during the implantation plays the important role for the formation of the Fe0 state while post annealing results in the preferential formation of Fe oxidation states, mainly Fe3+ states. The largest fraction of iron with Fe0 state was obtained at implantation temperatures between 600 and 800 °C.

[1] A. Perez, G. Marest, B.D. Sawicka, J.A. Sawicki, T. Tyliszczak, Phys. Rev. B, 28, 1227 (1983)