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

The electronic structure and magnetic properties of cobalt-doped (7.5%) and manganese co-doped (2.5%) ZnO polycrystalline samples have been investigated to understand the mechanism of room temperature ferromagnetism (RTFM) 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 (P6(3)mc) ZnO structure. The X-ray photoelectron spectroscopy (XPS) results indicate that the Co and Mn atoms are in +2 oxidation states, which incorporate at the Zn2+ site, with no signature of metallic clusters. The Co-doped sample prepared in air displays a paramagnetic state while the sample annealed in Ar atmosphere shows a weak ferromagnetic ordering at 300 K. The co-doping of Mn further enhances the ferromagnetic ordering, indicating that Co and Mn ions play 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 show a huge increment in the magnetic moment, however, the Mn ions seem to stay passive towards the hydrogen induced magnetization. Notably, the samples reheated in air show suppression of the induced ferromagnetism (FM). The resistance measurements suggest that the additional carriers induced upon hydrogenation also play some role in mediating the exchange coupling. The O 1s XPS and the XRD results show clear evidence of oxygen depletion in the samples upon hydrogenation, followed by a recovery upon their reheating in air. The observed FM is explained in terms of composite effect of the oxygen vacancies and the carrier density. Our results point out that the ferromagnetic ordering could be switched between 'on' and 'off' by introducing (through hydrogenation) and by removing (through reheating in air) the oxygen vacancies in Co-doped ZnO.