Mechanism of defect induced ferromagnetism in undoped and Cr doped TiO2 nanorods/nanoribbons

We have studied the effect of doping concentrations, growth temperature and calcinations on the structural, optical and magnetic properties of the undoped and Cr doped TiO2 nanorods/nanoribbons (NR/NRb), in order to develop an improved understanding on the mechanism of room temperature (RT) ferromagnetism (FM) in these nanostructures. Both undoped and doped TiO2 NR/NRb exhibit RT FM and a ∼2.6 fold enhancement in magnetization is observed in 0.3% Cr doped TiO2 NR/NRb as compared to the undoped NR/NRb, and the magnetization increases considerably after vacuum annealing. However, no measureable FM is observed for the precursor TiO2 powder, despite the presence of high concentration of oxygen vacancies in it. On the other hand, the magnetization decreases at higher doping concentration (0.7% Cr) as compared to 0.3% Cr doped sample. Thus, our studies revealed that a simple oxygen vacancy and/or Cr ions alone cannot yield the RT FM in TiO2 nanostructures. We argue that the oxygen vacancy with appropriate charge redistribution and their orbital overlapping, and exchange interaction with the nearby unpaired 3d electron of Ti3+ in undoped TiO2 and/or unpaired 3d electrons of Cr3+ dopant with proper charge distribution and occupation inside the host lattice in Cr doped TiO2 nanostructures play the pivotal role for the ferromagnetic ordering and observed RTFM. The presence of oxygen vacancy related F+-center and Cr3+ are confirmed from the electron spin resonance and x-ray photoelectron spectroscopy measurements.