Instability of Ferromagnetic Nanoclusters in Fe Implanted Amorphous SiO2

A 460 nm thick amorphous SiO2 layer, formed on a Si (100) surface by air-annealing the Si substrate at 1100oC for 24 h, was implanted with 57Fe to a fluence of 1 x 1016/cm2 at room temperature and annealed at temperatures up to 1000oC. The implanted and annealed samples were studied by conversion electron Mössbauer spectroscopy (CEMS) and magnetization measurements.The CEMS spectra up to an annealing temperature of 600oC showed the presence of a singlet due to dispersed Fe ions and paramagnetic doublets with hyperfine parameters characteristic of Fe2+ and Fe3+. The spectrum after the 1000oC annealing was dominated (> 80%) by ferromagnetic sextets, the main components of which were sextets with a hyperfine field of 320(20) kOe and 264(20) kOe, showing the formation of Fe0 clusters, in agreement with previous observations. Magnetization measurements (m(H)) on the sample after the 1000oC annealing showed a small hysteresis at 4 K and saturation magnetization with zero hysteresis at room temperature, reached with application of small external field.The CEMS measurement on this sample was repeated after storing the sample under ambient conditions for a period of 6 months. The spectrum showed complete disappearance of the ferromagnetic sextets and the presence only of paramagnetic doublets due to Fe2+. Evidently progressive oxidation of the Fe clusters had occurred. Magnetization results confirm the paramagnetic transformation of the Fe clusters.