Fe doping effect on the structural, magnetic and surface properties of SnO2 nanoparticles prepared by a polymer precursor method

In this study the structural, magnetic and surface characterization of Fe-doped SnO2 nanopowders synthesized by a polymer precursor method is presented. The x-ray diffraction (XRD) data analysis shows the formation of rutile-type structure for all samples. For Fe-content up to 5.0 mol% lattice constants and unit cell volume values suggest substitutional solution of Fe3+- and Sn4+-ions in the SnO2 matrix and the likely generation of oxygen vacancies to account for charge compensation. Above 5.0 mol% Fe-content the entrance of Fe3+-ions into interstitial sites seems to be the dominant regime. Magnetic measurements confirm the ferric valence state and suggest the coexistence of weak ferromagnetic (FM) with strong paramagnetic (PM) phases. Using the bound magnetic polaron (BMP) model the FM contribution has been associated to electrons trapped within oxygen vacancies (donor electrons) that form BMPs which overlap to create a spin-split impurity band. Despite the small size of the particles no evidence of thermal relaxation effects has been observed, which was assigned to the formation of aggregates of strongly interacting naked particles. Above ≈1.0 mol% Fe-content, the antiferromagnetic (AFM) interaction associated to Fe-clusters seems to be dominant and only a PM phase is observed. These results are consistent with XPS data analysis which indicates that the magnetic properties are strongly correlated with the surface properties of the particles.