Correlation between magnetic properties and lattice site location of Fe implanted TiO2 at different temperetures

Diluted magnetic semiconductors are materials which exhibit ferromagnetic and semiconducting properties simultaneously and are of great interest for spin injection. An interesting candidate is the transition metal doped TiO2 which has been reported to be ferromagnetic above room temperature (RT). It was found that nanometer-sized precipitates substantially contribute to the ferromagnetic properties.
In this study, rutile TiO2(110) single crystals were implanted with 100 keV 57Fe ions at 623 K with fluences between 1x10^16 and 4x10^16 cm^-2 and subsequently annealed at temperatures up to 1073 K. The maximum Fe concentration at the projected range Rp = 50 nm is about 8 at.%.
Virgin, implanted and post-annealed samples were investigated using conversion electron Mößbauer spectroscopy, channeling Rutherford backscattering spectrometry, x-ray diffraction, and super-conducting quantum interference device magnetometry.
Crystalline, imperfect bcc-Fe and FeO-TiO2 composites were detected already in the as-implanted state in the sample implanted at 623 K with 4x10^16 cm^-2. Besides secondary phases a small fraction of Fe^2+ is observed both in the as-implanted and at 923 K annealed sample and can be ascribed to Fe substituting Ti sites.
During annealing at 923 K, both phases grow in grain size, while the amount of FeO-TiO2 is drastically increased. After annealing at 1073 K, FeO-TiO2 composites are the predominant precipitates at the cost of metallic Fe. Both, bcc-Fe and FeO-TiO2 composites are textured. According to the CEMS and SQUID measurements the origin of ferromagnetism is attributed to bcc Fe clusters. The oxidization of metallic Fe with increasing annealing temperature results in the decrease of the saturation moment.
In order to prevent the formation of clusters another set of samples was implanted at RT and 230 K with same fluences. No cluster formation but also no ferromagnetism was observed in both cases.