Local structure and magnetic behaviour of Fe-doped TiO2 anatase nanoparticles:: experiments and calculations

We present an investigation of Fe-doped TiO2 anatase nanoparticles (2.8 and 5.4 at.% Fe) where Fe substitutes Ti atoms without the presence of other phases. In order to characterize these samples we used x-ray absorption experiments, Fe-57 Mossbauer spectroscopy, ab initio calculations and magnetometry. Results from iron K-edge near-edge and extended x-ray absorption fine structure confirm that Fe3+ replaces Ti4+ in the TiO2 anatase structure increasing the metal-anion bond length. Mossbauer spectra recorded at room temperature show asymmetric Fe3+ broad doublets. These results agree with structural, hyperfine and magnetic properties calculated using density-functional theory, if oxygen vacancies are present in the iron-oxygen octahedra. Mossbauer and magnetic measurements indicate that samples are paramagnetic at room temperature. At low temperatures, two kind of magnetic species can be distinguished: (i) isolated paramagnetic Fe3+ ions and (ii) antiferromagnetically coupled Fe3+ ions. These results also show that substitutional Fe in nanosized anatase TiO2 does not induce ferromagnetic ordering.