Magnetic moment orientation and in-depth distribution of dysprosium near the surface of DyCo4.6 thin films from x-ray circularly polarized absorption

We have investigated the dysprosium distribution and its magnetic moment orientation at the region near the surface of ${\mathrm{DyCo}}_{4.4}$ and ${\mathrm{DyCo}}_{4.6}$ ferrimagnetic amorphous films with perpendicular magnetic anisotropy. X-ray magnetic circular dichroism spectroscopy of the films at the Dy ${M}_{4,5}$ and Co ${L}_{2,3}$ edges using total electron yield (TEY) detection was performed at 2 K and 300 K temperatures, and at sample orientations ranged from ${0}^{\ensuremath{\circ}}$ to ${70}^{\ensuremath{\circ}}$ with respect to the normal to the sample. The measurements showed an apparent partial decoupling between the cobalt and dysprosium magnetic sublattices. At RT, the magnetic moment per atom of dysprosium was below the minimum value expected if all dysprosium moments were Antiferromagnetic (AF) coupled to cobalt. At 2 K, the cobalt sublattice presented a surprisingly stronger magnetic anisotropy than the dysprosium sublattice. A detailed analysis of the circularly polarized spectra of the Dy ${M}_{5}$ edge, based on the deconvolution of the spectra in their related parallel, antiparallel, and transverse to ${J}_{z}$ spectral components, demonstrates that the spectra are composed by dysprosium with different magnetic moment distributions. The fit of the Dy ${M}_{5}$ spectra using the ${J}_{z}$ spectral components evidenced a gradation of dysprosium concentration due to segregation at the region probed by TEY. The topmost layer was magnetically uncoupled from cobalt. At RT, $25%$ of the dysprosium magnetic moments in the underlayer were found averaged oriented in the same direction of cobalt. The expected weak magnetic coupling of these dysprosium atoms to cobalt should explain the surprisingly lower magnetic anisotropy of the dysprosium sublattice compared to that of cobalt probed by TEY at 2 K.