Abstract
Experimental studies of as thin films or nanocrystals have found ferromagnetism and Curie temperatures above room temperature and that - or -type doping of can change its magnetic state. Bulk with a low defect density and in the range used in experimental thin-film studies exhibits ferromagnetism only at very low temperatures. Therefore defects in thin-film samples or nanocrystals may play an important role in promoting magnetic interactions between Co ions in . The mechanism of exchange coupling induced by defect states is considered and compared to a model for ferromagnetism in dilute magnetic semiconductors [T. Dietl et al., Science 287, 1019 (2000)]. The electronic structures of Co substituted for Zn in ZnO, Zn, and O vacancies, substituted N, and interstitial Zn in ZnO were calculated using the B3LYP hybrid density functional in a supercell. The B3LYP functional predicts a band gap of for bulk ZnO, close to the experimental value of . Occupied minority-spin Co levels are at the top of the valence band and unoccupied levels lie above the conduction-band minimum. Majority-spin Co levels hybridize strongly with bulk ZnO states. The neutral O vacancy defect level is predicted to lie deep in the band gap, and interstitial Zn is predicted to be a deep donor. The Zn vacancy is a deep acceptor, and the acceptor level for substituted N is at midgap. The possibility that - or -type dopants promote exchange coupling of Co ions was investigated by computing the total energies of magnetic states of ZnO supercells containing two Co ions and an oxygen vacancy, substituted N, or interstitial Zn in various charge states. The neutral N defect and the singly positively charged O vacancy are the only defects which strongly promote ferromagnetic exchange coupling of Co ions at intermediate range. Total energy calculations on supercells containing two O vacancies and one Zn vacancy clearly show that pairs of singly positively charged O vacancies are unstable with respect to dissociation into neutral and doubly positively charged vacancies; the oxygen vacancy is a “negative ” defect. This apparently precludes simple charged O vacancies as a mediator of ferromagnetism in .
- Received 5 December 2005
DOI:https://doi.org/10.1103/PhysRevB.74.144432
©2006 American Physical Society