  In this paper we show that ferromagnetism can be induced in pure TiO2 single
crystals by oxygen ion irradiation. By combining x-ray diffraction,
Raman-scattering, and electron spin resonance spectroscopy, a defect complex,
\emph{i.e.} Ti$^{3+}$ ions on the substitutional sites accompanied by oxygen
vacancies, has been identified in irradiated TiO2. This kind of defect complex
results in a local (TiO$_{6-x}$) stretching Raman mode. We elucidate that
Ti$^{3+}$ ions with one unpaired 3d electron provide the local magnetic
moments.
