Symmetry and Nonstoichiometry as Possible Origin of Ferromagnetism in Nanoscale oxides

TL;DR

This study uses density functional theory to suggest that nanoscale oxides can inherently exhibit ferromagnetism due to symmetry and nonstoichiometry, even without defects, explaining their unusual magnetic properties.

Contribution

It introduces a novel mechanism where perfect nanocrystals become magnetic due to size-induced nonstoichiometry and symmetry effects, without relying on intrinsic defects.

Findings

Magnetic states can occur in perfect nanocrystals due to size effects.

Long-range magnetic order can arise from symmetry and nonstoichiometry.

Explains ferromagnetism features in nanoscale oxides without defects.

Abstract

We show through density functional theory calculations that extended magnetic states can inherently occur in oxides as the size of the crystals is reduced down to the nanometer scale even when they do not explicitly include intrinsic defects. This is because in nanoscale systems crystallographically perfect crystallites paradoxically result in nonstoichiometric compositions owing to the finite number of constituting atoms. In these structurally perfect but stoichiometrically imperfect nanocrystallites, the spin-triplet state is found to be more stable than the spin-singlet state, giving rise to an extended spin distribution that expands over the entire crystal. According to this picture, long-range magnetic order arises from the combined effect of crystal symmetry and nonstoichiometry that can coexist exclusively in nanoscale systems. The idea can also give reasonable explanations for the unprecedented ferromagnetic features observed commonly in nanoscale oxides, including ubiquity, anisotropy, and diluteness.