Microscopic grain structures and magnetic properties of Co-doped ZnO-patterned films were studied by combinatorial atomic force microscopy and magnetic force microscopy (MFM) imaging. Whereas Co-doped ZnO was not uniformly magnetized, Co ions were homogeneously doped in the films and substitute from the Zn sites. Most Co:ZnO grains were weakly ferromagnetic (FM) and surrounded by strong FM foamlike grain boundaries networks. No MFM contrast was observed in pure ZnO films grown under similar conditions. The findings clearly demonstrate that defective grain boundaries and magnetic doping have key roles FM properties of diluted magnetic oxides.
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