Linearly polarized X-ray absorption investigation of ultrathin NiOx/Pd(100) films

Linearly polarized soft X-ray absorption spectroscopy experiments both on Ni L2,3 and O K edges have been carried out on NiO ultrathin films on Pd(100), from the c(4 × 2) defective monolayer up to fully relaxed films. In the defective monolayer a dichroic effect on the Ni L2,3 edge has been detected, and it has been associated to anisotropic structural effects (e.g., local crystal field symmetry) due to the spatial confinement. This produces a change in the energy sequence of the Ni 3d levels resulting in different transition probabilities for the various levels at different polarization. A support to this interpretation has been also obtained by theoretical computations. In the case of thicker films, a clear dichroic effect, reversed with respect to the one observed for a bulk like NiO(100) surface, has been observed for the L2 white line at all the explored thicknesses. This has been associated to a different spin structure of the ultrathin films, where the domains are preferentially aligned to the surface plane instead of being perpendicular to it. However, we have not observed the inversion of dichroism even for very thick fully relaxed films. The dependence of the dichroic effect on the preparation procedure of the films has been also explored. For the thicker films the dichroic effect is independent from the actual oxygen pressure used during the growth, while remarkable differences are found in the case of the thinner films.