Activation of high-Tc ferromagnetism in Co2+:TiO2 and Cr3+:TiO2 nanorods and nanocrystals by grain boundary defects

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 clopant or defect migration are not the dominating factors converting the doped TiO2 nanocrystals from the paramagnetic state to the ferromagnetic state. The most important factor for activating ferromagnetism is found to be the creation of grain boundary defects, proposed to be oxygen vacancies at nanocrystal fusion interfaces. These defects are passivated and the ferromagnetism destroyed by further aerobic annealing. These results not only help elucidate the origins of the TMn+:TiO2 DMS ferromagnetism but also represent an advance toward the controlled manipulation of high-T-C DMS ferromagnetism using external chemical perturbations.