We report ferromagnetism above 300 K in ZnO:xCu (x in at. ) sputtered thin films. For x < 1, a large magnetic moment of 1.6 mu(B)/Cu was observed, which decreases monotonically with increasing x. We find evidence that the ferromagnetic moment is due to Cu-O planar nanophase inclusions in ZnO basal planes. The presence of CuO nanophase is confirmed by transmission electron microscopy, x-ray diffraction, and Raman spectroscopy studies. These inclusions are present even for x < 3, where Cu-O structures of a few nanometers in size are observed. Field-cooled and zero-field-cooled magnetization measurements show a bifurcation for temperatures below 300 K.
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