Synthesis and room-temperature ferromagnetism of pure and Cu-doped SnO2 nanowires grown by thermal evaporation

We report ferromagnetism in pure and Cu-doped tin oxide (SnO2) nanowires grown on silicon substrate using a thermal evaporation process at atmospheric pressure. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images indicated the growth of wire-like nanostructures on silicon substrate with average diameter of 30-110 nm and length of 60-100 µm. The EDX spectra confirmed the growth of pure and Cu-doped SnO2 nanowires with Cu concentration of 0.25 at%, 0.5 at% and 2 at%. X-ray diffraction study revealed that as-grown nanowires have a tetragonal rutile structure of SnO2, without any secondary phase and were polycrystalline in nature. The room temperature photoluminescence (PL) spectra of as-grown nanowires showed strong emission at 590 nm and 600 nm originated due to oxygen vacancies and defect-related electronic states in the band gap. The ferromagnetism was observed in the pure and Cu-doped SnO2 nanowires at room temperature as well as at lower temperature of 5 K. At room temperature (300 K), pure SnO2 nanowires were ferromagnetic with a very small saturation magnetization (Ms) of ~0.06 emu/g, a remanent magnetization (Mr) of ~16.67% of Ms, and a coercivity of ~101 Oe while the nanowires doped with 0.25 at%, 0.5 at% and 2 at% of Cu were ferromagnetic with a saturation magnetization of ~10.25 emu/g, ~11.64 emu/g and 15.65 emu/g, a remanent magnetization of ~31.70%, ~46.13% and 56.29% of Ms, and a coercivity of ~162 Oe, ~181 Oe and 273 Oe, respectively. The magnetization of as-grown nanowires increases from 0.06 emu/g to 15.65 emu/g with increase in Cu-dopant concentration from 0 at% to 2 at%. Electron Spin Resonance (ESR) spectra also confirmed the room temperature ferromagnetism in Cu-doped SnO2 nanowires. The origin of room temperature ferromagnetism can be attributed to both surface defects and oxygen vacancies.