X-ray dichroism as a probe of the electronic ground state in ultrathin rare-earth overlayers

We present polarization- and temperature-dependent Tb ${\mathit{M}}_5$ x-ray-absorption total-yield spectra of Tb submonolayer overlayers on a Ni(110) surface. Strong polarization dependencies are observed. This dichroism is found to decrease with layer thickness, increase upon annealing, and, for annealed layers, increase upon cooling. Although the observed spectral shapes can be reproduced with an atomic calculation including a magnetic field only, we suspect (surface) crystal fields may play an essential role.