Colloidal Co2+- and Cr3+-doped TiO2 nanorods and nanocrystals were synthesized and studied by X-ray powder diffraction, electronic absorption spectroscopy, magnetic circular dichroism spectroscopy, magnetic susceptibility, and transmission electron microscopy. The nanorods were paramagnetic as colloids but showed room-temperature ferromagnetism when spin-coated aerobically into films. Crystalline domain size, thermal annealing, and dopant or defect migration were not the dominating factors converting the doped TiO2 nanocrystals from the paramagnetic state to the ferromagnetic state. The most important factor for activating ferromagnetism was found to be the creation of grain boundary defects, proposed to be O vacancies at nanocrystal fusion interfaces. These defects were passivated and the ferromagnetism destroyed by further aerobic annealing. These results not only help elucidate the origins of the TM(n+):TiO2 DMS ferromagnetism but also represented an advance towards the controlled manipulation of high-TC DMS ferromagnetism using external chemical perturbations.

Activation of High-TC Ferromagnetism in Co2+:TiO2 and Cr3+:TiO2 Nanorods and Nanocrystals by Grain Boundary Defects. Bryan, J.D., Santangelo, S.A., Keveren, S.C., Gamelin, D.R.: Journal of the American Chemical Society, 2005, 127[44], 15568-74