Strong Room-Temperature Ferromagnetism in Co2+-Doped TiO2 made from Colloidal Nanocrystals

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.