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-L-2,L-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 T-C, a key quantity for magnetism, amounts to only half the bulk value of similar to 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, parallel to L-3/L-2 parallel to, is up to three times higher than the value expected from the degeneracy of the 2p(3/2) and 2p(1/2) core states, which is observed for the bulklike layers. This is mainly due to a reduced L-2 XMCD amplitude. We suggest that the effect may be related to the different weight the crystal-field split Gd-5d final states (t(2g) and e(g)) have in the absorption process at the L-2 and L-3 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 L-2 absorption edge is shifted to lower energies upon ferromagnetic ordering while the L-3-edge position remains inert.