  Colloidal cobalt-doped TiO2 (anatase) nanocrystals were synthesized and
studied by electronic absorption, magnetic circular dichroism, transmission
electron microscopy, magnetic susceptibility, cobalt K-shell X-ray absorption
spectroscopy, and extended X-ray absorption fine structure measurements. The
nanocrystals were paramagnetic when isolated by surface-passivating ligands,
weakly ferromagnetic (Ms = 1.5 x 10-3 mB/Co2+ at 300 K) when aggregated, and
strongly ferromagnetic (up to Ms = 1.9 mB/Co2+ at 300 K) when spin-coated into
nanocrystalline films. X-ray absorption data reveal that cobalt is in the Co2+
oxidation state in all samples. In addition to providing strong experimental
support for the existence of intrinsic ferromagnetism in cobalt-doped TiO2,
these results demonstrate the possibility of using colloidal TiO2 diluted
magnetic semiconductor nanocrystals as building blocks for assembly of
ferromagnetic semiconductor nanostructures with potential spintronics
applications.
