Defect-Induced Room-Temperature Ferromagnetism in ${\rm SnO}_{2}$ Nanowires Controlled by UV Light Irradiation

Nonferromagnetic
${\rm SnO}_{2}$
nanowires show ferromagnetism at room temperature, which is significantly enhanced upon irradiation by ultraviolet (UV) light of wavelength 365 nm. The structural characterizations on as-grown, UV-irradiated, and
${\rm O}_{2}$
-annealed samples show no measurable difference in X-ray diffraction profiles and field-emission scanning and transmission electron microscopy images. But optical absorption, photoluminescence, and X-ray photoelectron spectroscopy reveal a significant effect of UV irradiation and establish a remarkable enhancement of O vacancies in
${\rm SnO}_{2}$
nanowires upon UV light irradiation, which diminishes after annealing in the presence of
${\rm O}_{2}$
gas. The as-grown and
${\rm O}_{2}$
-annealed nanowires show weak ferromagnetism at room temperature, but the UV-irradiated nanowires show strong ferromagnetism. The origin of such ferromagnetism in pure (undoped)
${\rm SnO}_{2}$
is attributed to the presence of oxygen vacancies, which is tunable externally using UV irradiation.