Occurrence and stability of ferromagnetism in chemically synthesized cobalt doped TiO ₂

We report the development of ferromagnetism in ∼30 nm sized well-characterized Ti _1-xCo _xO ₂ powders with x = 0.00015-0.006 and its absence for x > 0.006. In addition, these studies show the effect of Co doping on the structural stability and anatase to rutile phase transformation. X-ray diffraction data of samples synthesized by a wet chemical method and annealed at 450 °C indicate a limited solubility of ∼1.2% for Co in the anatase TiO ₂ matrix, and with further increase, the CoTiO ₃ phase is formed along with increased presence of rutile TiO ₂. The bandgap (∼3.23 eV) of the anatase TiO ₂ remained almost unchanged for x < 0.006, but decreased rapidly for x < 0.006 approaching 2.8 eV for x = 0.03. The magnetic data from Ti _1-xCo _xO ₂ samples with x = 0.006 showed a coercivity H _c ∼ 150 Oe and a weak magnetic moment of 0.2 μ _B/ion at 300 K. The ferromagnetism of Ti _0.994Co _0.006O ₂ with open hysteresis loops continue up to a high superparamagnetic blocking temperature T _B ∼ 675 K, above which a superparamagnetic behavior was observed. Systematic changes in the structural, magnetic and optical properties suggest that Co doping is an excellent method to tailor the physical properties of TiO ₂ nanoparticles.