Ferromagnetism as a universal feature of nanoparticles of the otherwise nonmagnetic oxides

TLDR

The study proposes that ferromagnetism is a universal property of metal‑oxide nanoparticles. The authors attribute the ferromagnetism to exchange interactions among surface‑localized spin moments created by oxygen vacancies, rather than magnetic impurities. Nanoparticles of nonmagnetic oxides exhibit room‑temperature ferromagnetism with saturated moments comparable to doped oxides, while bulk sintered samples lose magnetism.

Abstract

Room-temperature ferromagnetism has been observed in nanoparticles $(7--30\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\phantom{\rule{0.2em}{0ex}}\mathrm{diam})$ of nonmagnetic oxides such as ${\mathrm{CeO}}_{2}$, ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, $\mathrm{ZnO}$, ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$, and ${\mathrm{SnO}}_{2}$. The saturated magnetic moments in ${\mathrm{CeO}}_{2}$ and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ nanoparticles are comparable to those observed in transition-metal-doped wideband semiconducting oxides. The other oxide nanoparticles show somewhat lower values of magnetization but with a clear hysteretic behavior. Conversely, the bulk samples obtained by sintering the nanoparticles at high temperatures in air or oxygen became diamagnetic. As there were no magnetic impurities present, we assume that the origin of ferromagnetism may be the exchange interactions between localized electron spin moments resulting from oxygen vacancies at the surfaces of nanoparticles. We suggest that ferromagnetism may be a universal characteristic of nanoparticles of metal oxides.