Phase change in Fe implanted rutile TiO2 after thermal treatment

Diluted magnetic semiconductors (DMS) are materials which simultaneously exhibit ferromagnetic and semi-conducting properties by substitution of transition metal atoms onto cation sites. Recently, transition metal doped TiO2 has been reported to be ferromagnetic above room temperature. It was found that nanoscaled precipitates can substantially contribute to the ferromagnetic properties.
In this study, commercial rutile TiO2(110) single crystals were implanted with 180 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 projected range Rp = 89 nm was about 5 %.
Virgin, implanted and post-annealed samples were investigated using conversion electron Mößbauer spectroscopy, channeling Rutherford backscattering spectrometry (C-RBS), synchrotron-XRD, and super-conducting quantum interference device (SQUID) magnetometry.
Crystalline, imperfect bcc-Fe and FeO-TiO2 composites (or solid solution) were detected in the sample implanted with 4x10^16 cm^-2 already in the as-implanted state. Besides secondary phases a small fraction of Fe^2+ is observed 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-TiO^2 is drastically increased, accompanied by a change of the Fe site location. After annealing at 1073 K, FeO-TiO2 composites are the predominated 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 .