Unexpected room-temperature ferromagnetism in bulk ZnO.

It is demonstrated that a transition from paramagnetic behavior to clear room-temperature ferromagnetism (RTFM) exists in pure bulk ZnO. A significant enhancement of RTFM has been observed in argon-annealed ZnO samples. Quantitative chemical analysis unambiguously indicates that oxygen-related vacancies at surface play a crucial role in this observed RTFM. We suppose that the surface magnetic states, paramagnetic in the pure nanoparticles, are converted to ferromagnetic phase after mechanical compaction. Additionally, it is found that weakly adsorbed carbon species could block the exchange coupling between isolated magnetic moments in the surface layers. [ABSTRACT FROM AUTHOR]