Magnetic, electrical, and microstructural characterization of ZnO thin films codoped with Co and Cu

Abstract

Here we report on systematic studies of the epitaxial growth and properties of Co and Cu codoped ZnO thin films deposited onto sapphire c -plane single crystals using pulsed-laser deposition. The films display ferromagnetic behavior at room temperature. Detailed atomic scale characterization rules out the presence of clusters and secondary phases as the source of ferromagnetism. Optical measurements and x-ray photoelectron spectroscopy confirm the direct substitution of dopant atoms into Zn lattice sites. At low concentrations of Cu (∼5%) the magnetic moment of Zn1-(0.05+x) Co0.05 Cux O materials appears to be additive. At higher concentrations of Cu the net magnetic moment per atom drops off sharply and seems to be relatively insensitive to the Co content. There is a dramatic increase in resistivity of the Co-doped films that accompanies Cu doping. Yet, this change of resistivity does not affect the magnetic moment, suggesting that free carrier mediated mechanism is not a feasible explanation for ferromagnetism in these films. The known presence of high oxygen vacancies in these films does allow for possible defect mediated mechanisms (e.g., bound magnetic polarons) for mediating exchange coupling of the dopant (Co,Cu) ions resulting in room temperature ferromagnetism. © 2007 American Institute of Physics.