Theory of Ferromagnetism in Reduced ZrO_2–<i>x</i> Nanoparticles

Abstract

Bulk ZrO₂ is both nonreducible and nonmagnetic. Recent experimental results show that dopant-free, oxygen-deficient ZrO_2-<i>x</i> nanostructures exhibit a ferromagnetic behavior at room temperature (RT). Here, we provide a comprehensive theoretical foundation for the observed RT ferromagnetism of zirconia nanostructures. ZrO₂ nanoparticles containing up to 700 atoms (3 nm) have been studied with the help of density functional theory. Oxygen vacancies in ZrO₂ nanoparticles form more easily than in bulk zirconia and result in electrons trapped in 4d levels of low-coordinated Zr ions. Provided the number of these sites exceeds that of excess electrons, the resulting ground state is high spin and the ordering is ferromagnetic. The work provides a general basis to explain magnetism in intrinsically nonmagnetic oxides without the help of dopants.