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

Room-temperature ferromagnetism has been observed in nanoparticles $(7–30\mathrm{nm}\mathrm{diam})$ 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.

Figure 1

(Color online) FESEM images of ${\mathrm{CeO}}_2$ nanoparticles [(a) $7\mathrm{nm}$, (b) $15\mathrm{nm}$, (c) $500\mathrm{nm}$] and their magnetization curves at $300\mathrm{K}$. Note the absence of ferromagnetism in the $500\mathrm{nm}$ nanoparticles, in contrast to the 7 or $15\mathrm{nm}$ nanoparticles.

Figure 2

(Color online) (a) TEM image of ${\mathrm{Al}}_2{\mathrm{O}}_3$ nanoparticles heated at $500°\mathrm{C}$ and (b) their magnetization curves showing ferromagnetism even at $390\mathrm{K}$. Note that the nanoparticles sintered at $1400°\mathrm{C}$ exhibit diamagnetic behavior at $300\mathrm{K}$.

Figure 3

(Color online) $M$ versus $H$ curves measured at $300\mathrm{K}$ for nanoparticles of $\mathrm{ZnO}$ heated at $400°\mathrm{C}$ and sintered at $1200°\mathrm{C}$.

Figure 4

(Color online) $M$ versus $H$ curves measured at $300\mathrm{K}$ for nanoparticles of ${\mathrm{In}}_2{\mathrm{O}}_3$ and ${\mathrm{SnO}}_2$ heated at $400°\mathrm{C}$ and sintered at $1200°\mathrm{C}$.