We have studied the electronic structure and the magnetism of Cu-doped ZnOnanowires, which have been reported to show ferromagnetism at room temperature[G. Z. Xing ${et}$ ${al}$., Adv. Mater. {f 20}, 3521 (2008).], by x-rayphotoemission spectroscopy (XPS), x-ray absorption spectroscopy (XAS) and x-raymagnetic circular dichroism (XMCD). From the XPS and XAS results, we find thatthe Cu atoms are in the "Cu$^{3+}$" state with mixture of Cu$^{2+}$ in the bulkregion ($sim$ 100 nm), and that "Cu$^{3+}$" ions are dominant in the surfaceregion ($sim$ 5 nm), i.e., the surface electronic structure of the surfaceregion differs from the bulk one. From the magnetic field and temperaturedependences of the XMCD intensity, we conclude that the ferromagneticinteraction in ZnO:Cu NWs comes from the Cu$^{2+}$ and "Cu$^{3+}$" states inthe bulk region, and that most of the doped Cu ions are magnetically inactiveprobably because they are antiferromagnetically coupled with each other.
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