Oxygen vacancy-mediated enhanced ferromagnetism in undoped and Fe-doped TiO_2 nanoribbons

摘要

We have investigated the structural, optical and ferromagnetic properties of undoped and Fe-doped TiO_2 nanoribbons (NRbs) grown by a solvothermal method. A strong room temperature ferromagnetism (RTFM) is observed in both undoped and Fe-doped TiO_2 NRbs. Fe-doped TiO_2 NRbs exhibited a ～4.8-fold enhancement in RTFM as compared to the undoped NRbs grown under similar conditions. However, the RTFM decreases at higher Fe concentration, possibly due to antiferromagnetic ordering between nearby Fe~(3+) ions caused by a super exchange interaction. X-ray diffraction patterns reveal the pure TiO_2(B) phase, the TiO_2(B)–anatase mixed phase and the anatase–rutile mixed phase of the TiO_2 structure. Field emission scanning electron microscopy and transmission electron microscopy observations reveal NRbs with uniform pore distribution and nanopits formed on the surface for both undoped and Fe-doped NRbs. These samples exhibit strong visible photoluminescence associated with oxygen vacancies and the ferromagnetic hysteresis loop, both of which are strongly enhanced after vacuum annealing. Optical absorption, electron spin resonance and x-ray photoelectron spectroscopic analyses are performed to elucidate the origin of RTFM. The observed RTFM in undoped and Fe-doped TiO_2 NRbs is qualitatively explained through a model involving bound magnetic polarons, which include an electron locally trapped by an oxygen vacancy with the trapped electron occupying an orbital overlapping with the unpaired electron (3d~1) of a Ti~(3+) ion and/or the unpaired electron (3d~5) of a Fe~(3+) ion. The development of TiO_2 NRbs with tunable optical and magnetic properties constitutes an important step towards realizing improved magneto-optical and spintronic devices from novel TiO_2 nanostructures.