Surface magnetization reversal of sputtered CrO2

We have investigated the magnetic behavior of semi-metallic CrO2 powder samples as a function of Ar sputtering at low kinetic energy. By the use of X-ray absorption spectroscopy at the O K edge and X-ray magnetic circular dichroism at the Cr L-2,L-3 edges as a function of sputtering time, a strong decrease of the absolute magnetic moment has been observed corresponding to a surface reduction to a Cr2O3-like compound, deduced by chemical shifts in the X-ray absorption spectra. An unexpected change of sign of the Cr L-2,L-3 dichroism signal was found and interpreted by the formation of an antiparallel coupled ferromagnetic top layer. By comparison with epitaxial grown CrO2 films a quantitative estimation of the initial Cr2O3 toplayer thickness is possible.