| Abstract
| - Colloidal cobalt-doped TiO2 (anatase) nanocrystals were synthesized and studied by electronicabsorption, magnetic circular dichroism, transmission electron microscopy, magnetic susceptibility, cobaltK-shell X-ray absorption spectroscopy, and extended X-ray absorption fine structure measurements. Thenanocrystals were paramagnetic when isolated by surface-passivating ligands, weakly ferromagnetic (Ms≈ 1.5 × 10-3 μB/Co2+ at 300 K) when aggregated, and strongly ferromagnetic (up to Ms = 1.9 μB/Co2+ at300 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 ofintrinsic ferromagnetism in cobalt-doped TiO2, these results demonstrate the possibility of using colloidalTiO2 diluted magnetic semiconductor nanocrystals as building blocks for assembly of ferromagneticsemiconductor nanostructures with potential spintronics applications.
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