Size, Surface Structure, and Doping Effects on Ferromagnetism in SnO₂

The effects of crystallite size, surface structure, and dopants on the magnetic properties of semiconducting oxides are highly controversial. In this work, Fe:SnO₂ nanoparticles were prepared by four wet-chemical methods, with Fe concentration varying from 0% to 20%. Analysis confirmed pure single-phase cassiterite with a crystallite size of 2.6 ± 0.1 nm that decreased with increasing. Fe% doped substitutionally as Fe³⁺. Pure SnO₂ showed highly reproducible weak magnetization that varied significantly with synthesis method. Interestingly, doping SnO₂ with Fe −4 µ_B/Fe ion at 0.1% Fe doping was much larger than the 2.6 × 10⁻⁶ µ_B/Fe ion of pure Fe oxide nanoparticles synthesized under similar conditions. At Fe ≥ 2.5%, the magnetic moment was significantly reduced. This work shows that (1) pure SnO₂ can produce an intrinsic ferromagnetic behavior that varies with differences in surface structure, (2) very low Fe doping results in high magnetic moments, (3) higher Fe doping reduces magnetic moment and destroys ferromagnetism, and (4) there is an interesting correlation between changes in magnetic moment, bandgap, and lattice parameters. These results support the possibility that the observed ferromagnetism in SnO₂ might be influenced by modification of the electronic structure by dopant, size, and surface structure.