Effects of the Inhomogenous Co Doping on the Magnetoresistance of Zn1-xCoxO Epitaxial Films

A series of Zn1-xCoxO epitaxial films around 100 nm with nominal Co concentration from 5% to 15% was prepared by ultra high vacuum (UHV) magnetron reactive sputtering. The optical, magnetic and magneto-transport properties of this series of Zn1-xCoxO epitaxial films were investigated, respectively. Resonant Raman spectra indicate the high structural and crystalline quality of these Zn1-xCoxO (5 <= x <= 15%) films, and confirm a consistent correlation between the substituting Co ions content with the Co doping concentration as well. Paramagnetism, superparamagnetism and ferromagnetism with altered Curie temperature from low temperatures to above room temperatures have been observed in these films by SQUID nnagnetometry. The broad blocking temperature range indicates the presence of inhomogenous distribution of the magnetic nano-clusters in the superparamagnetic films. However, the magneto-transport behaviors do not strongly respond to the change of the magnetic properties from paramagnetism to ferromagnetism of these Zn1-xCoxO films. The lack of efficient coupling between the inhomogenous magnetic nanoclusters and the carrier system in ferromagnetic Zn1-xCoxO films highlights the absence of the intrinsic magnetic origins in high structural quality Zn1-xCoxO (5 <= x <= 15%) epitaxial films. On the other hand, the competition between the spin alignments and the inhomogenous local disorder effect by magnetic ions is suggested to be responsible for the carrier properties and the oberseved magnetoresistance in these Co doping Zn1-xCoxO (5 <= x <= 15%) epitaxial films.