The surface of ferromagnetic Zn1-xCoxO wurtzite epilayers has been studied by coupling atomic force microscopy and advanced x-ray spectroscopy. It was found that, even in high-quality epilayers, the formation of Co clusters and iso-space-group Co-rich regions can take place at the sample surface while the bulk maintains random Co distribution. Comparing structural characterization with magnetometry, it was shown that these surface modifications are not at the origin of the magnetic properties of the material. Quite the reverse, ferromagnetic behavior is enhanced in the sample characterized by the less defective surface.
Figure: (a) X-ray reflectivity curve of sample A (Co = 6%, bottom curve) and B (Co = 4%, top curve), circles indicates the grazing angles at which XANES spectra have been taken: the smaller is the angle the higher surface sensitivity is obtained; the fraction of Co atoms embedded in different phases as a function of sample depth is reported in the inset table. (b) Co K-edge XANES spectra recorded at different incidence angles around the critical one along with the relative best fits.