The electronic structure and magnetic properties of cobalt-doped (7.5%) and manganese co-doped (2.5%) ZnO polycrystalline samples were investigated to understand the mechanism of room-temperature ferromagnetism in dilute magnetic semiconductors. The samples in powder form were annealed in argon and hydrogen atmospheres followed by their repressing into pellets and reheating in air. Rietveld analysis of X-ray diffraction patterns confirmed the single-phase nature of the samples in the wurtzite type hexagonal (P63mc) ZnO structure. The X-ray photo-electron spectroscopy results indicated that the Co and Mn atoms were in +2 oxidation states, which incorporate at the Zn2+ site, with no signature of metallic clusters. The Co-doped sample prepared in air displayed a paramagnetic state while the sample annealed in Ar atmosphere showed a weak ferromagnetic ordering at 300K. The co-doping of Mn further enhances the ferromagnetic ordering, indicating that Co and Mn ions played an additive role in inducing the ferromagnetic ordering in the ZnO matrix. Interestingly, the Co- and (Co + Mn)-doped ZnO samples annealed in hydrogen atmosphere showed a huge increment in the magnetic moment, however, the Mn ions seemed to stay passive towards the hydrogen induced magnetization. Notably, the samples reheated in air showed suppression of the induced ferromagnetism. The resistance measurements suggested that the additional carriers induced upon hydrogenation also played some role in mediating the exchange coupling. The O 1s X-ray photo-electron spectroscopic and the X-ray diffraction results showed clear evidence of oxygen depletion in the samples upon hydrogenation, followed by a recovery upon their reheating in air. The observed ferromagnetism was explained in terms of composite effect of the oxygen vacancies and the carrier density. The results indicated that the ferromagnetic ordering could be switched between ‘on’ and ‘off’ by introducing (through hydrogenation) and by removing (through re-heating in air) the oxygen vacancies in Co-doped ZnO.

Switch ‘On’ and ‘Off’ Ferromagnetic Ordering Through the Induction and Removal of Oxygen Vacancies and Carriers in Doped ZnO: a Magnetization and Electronic Structure Study. R.K.Singhal, A.Samariya, S.Kumar, Y.T.Xing, U.P.Deshpande, T.Shripathi, S.N.Dolia, E.B.Saitovitch: Physica Status Solidi A, 2010, 207[10], 2373–86