Using the spectroscopies based on x-ray absorption, we have studied the structural and magnetic properties of ${\mathrm{Zn}}_{1-x}{\mathrm{Co}}_x\mathrm{O}$ films ($x=0.1$ and 0.25) produced by reactive magnetron sputtering. These films show ferromagnetism with a Curie temperature $T_C$ above room temperature in bulk magnetization measurements. Our results show that the Co atoms are in a divalent state and in tetrahedral coordination, thus substituting Zn in the wurtzite-type structure of ZnO. However, x-ray magnetic circular dichroism at the $\mathrm{Co}{L}_{2,3}$ edges reveals that the $\mathrm{Co}3d$ sublattice is paramagnetic at all temperatures down to $2\mathrm{K}$, both at the surface and in the bulk of the films. The $\mathrm{Co}3d$ magnetic moment at room temperature is considerably smaller than that inferred from bulk magnetization measurements, suggesting that the $\mathrm{Co}3d$ electrons are not directly at the origin of the observed ferromagnetism.

• Received 7 December 2006

DOI:https://doi.org/10.1103/PhysRevB.76.125201