X-ray magnetic circular dichroism at the Gd L2,3 absorption edges in GdN layers: The influence of lattice expansion

We have measured core-level x-ray-absorption spectra and x-ray magnetic circular dichroism (XMCD) at the Gd- L2,3 edges to characterize the low-lying Gd-5d derived conduction-band states in thin films of the 4f ferromagnet GdN with a unit-cell volume 8.6% above that of bulklike layers. The nonequilibrium structure is obtained by N+ plasma-assisted reactive sputter deposition at room temperature. The Curie temperature TC, a key quantity for magnetism, amounts to only half the bulk value of ∼60 K indicating a significant reduction of the effective exchange interaction between the 4f states. An intricate observation is that the ratio of the dichroic signal amplitudes in the lattice-expanded layers, L3 L2 , is up to three times higher than the value expected from the degeneracy of the 2 p3 2 and 2 p1 2 core states, which is observed for the bulklike layers. This is mainly due to a reduced L2 XMCD amplitude. We suggest that the effect may be related to the different weight the crystal-field split Gd-5d final states (t2g and eg) have in the absorption process at the L2 and L3 edges, and to the special electronic band structure of this strongly correlated material and its modification upon lattice expansion. This hypothesis is supported by the observation that the L2 absorption edge is shifted to lower energies upon ferromagnetic ordering while the L3 -edge position remains inert.