Paramagnetism of the Co sublattice in ferromagnetic Zn1-xCoxO films

Using the spectroscopies based on x-ray absorption, we have studied the structural and magnetic properties of Zn1-xCoxO 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 Co L-2,L-3 edges reveals that the Co 3d sublattice is paramagnetic at all temperatures down to 2 K, both at the surface and in the bulk of the films. The Co 3d magnetic moment at room temperature is considerably smaller than that inferred from bulk magnetization measurements, suggesting that the Co 3d electrons are not directly at the origin of the observed ferromagnetism.