Giant positive magnetoresistance in Co-doped ZnO nanocluster films

We have studied nanostructures and magnetoresistance of 0.5%, 12%, and 30% Co-doped ZnO nanocluster films which were deposited by a third generation sputtering-gas-aggregation source on Si wafer. Microstructure analysis is performed by x-ray photoelectron spectrometer, transmission electron microscopy, and x-ray diffraction and shows a uniform mean nanocrystallite size of 20nm with perfect wurtzite ZnO structure. Magnetoresistance (MR) increases at 5K with Co doping concentration, that is, 0.5% Co-doped ZnO with 469% MR, while the other two samples have 744% and 811%. The large positive MR is explained by the suppression of spin-dependent hopping paths when localized states with onsite correlation undergo a relatively large spin (Zeeman) splitting in a magnetic field due to strong s,p-d interactions in Co-doped ZnO nanocluster films.