Fabrication, characterization and studies of annealing effects on ferromagnetism inZn1−xCoxO nanowires

Abstract

Diluted magnetic semiconductorZn1−xCoxO (x≤0.11) nanowires withaverage diameter of ∼40 nm were prepared by thermal evaporation, followed by high-energy Co ionimplantation. Bombardment by Co ions produced a good number of structuraldefects (stacking faults and orientational variations) in the nanowires. Theas-implanted nanowires were paramagnetic. We performed two types ofthermal annealing, one in 1 atm argon flow and the other in a high vacuum, at600 °C, and studied the effects of annealing on the magnetic properties of these nanowires. Argonannealing removed structural defects in the nanowires and the nanowires then revealedferromagnetic ordering. This result suggests that structure defects are harmful to theoccurrence of ferromagnetism in the Co-implanted ZnO. The structure of the as-implantedand the annealed nanowires was inspected in detail by using scanning electron microscopy,energy dispersive x-ray spectroscopy, maps of electron energy loss spectra, x-ray diffraction,and high-resolution transmission electron microscopy. Taken together, these studiessuggested that no second phase existed on the scale down to the spatial resolution of∼0.5 nm. Noticeably, the nanowires even displayed largely enhanced ferromagnetism afterannealing in a high vacuum. A subsequent annealing in oxygen has also beenperformed on those vacuum-annealed nanowires to study the roles played by the Ovacancies in determining the ferromagnetic properties of the nanowires. Our resultsindicate that both the improved structural quality and the increased number ofO vacancies are key factors for the occurrence of ferromagnetic ordering in theZn1−xCoxO nanowires.