Giant positive magnetoresistance in Co-doped ZnO nanocluster films*

We studied magnetoresistance of 0.5{\%}, 12{\%} and 30{\%} Co-doped ZnO nanocluster films which are deposited on Si wafer. Microstructure analysis is performed by XPS, TEM and XRD, and shows a uniform mean size of 20 nm with perfect wurtzite ZnO structure. MR increases with Co doping concentration-0.5{\%} with 469{\%} while the other two samples have 744{\%} and 811{\%}. The large magnetoresistance is explained by suppression of spin-dependent hopping paths when localized states with onsite correlation undergo a relatively large Zeeman splitting in a magnetic field due to strong s,p-d interactions.