GdN thin films: Bulk and local electronic and magnetic properties

A study of high-quality thin films of the ferromagnet gadolinium nitride, GdN, is reported. The films, prepared by reactive ion-beam sputtering, show good stoichiometry and the lattice parameter, Curie temperature T_C, and saturation magnetization of the bulk material. The electrical conductivity is thermally activated down to the onset of magnetic ordering where there is evidence of a transition to metallic behavior. The transition can be tuned by a magnetic field, as reflected by a giant negative magnetoresistance. The ordered 4f moment extracted from the spectra of x-ray magnetic circular dichroism at the gadolinium M_4,5 edges is consistent with the ⁸S_7∕2 configuration of Gd³⁺; it varies with temperature as the macroscopic magnetization of the GdN layers. The experimental K-edge photoabsorption spectra of nitrogen in this compound indicate the presence of N p character of the low-lying unoccupied conduction-band states, pointing to hybridization of the N 2p and Gd (5d,6s) states. However, a comparison of the spectra with the theoretical partial density of vacant N p states shows considerable disparities that are not well understood. The exchange field generated by the Gd f electrons in the ferromagnetic phase of GdN induces a magnetic polarization of the N p band states, as can be concluded from the observation of strong magnetic circular dichroism at the K edge of nitrogen. It indicates the presence of an important spin-orbit interaction in the final N p states.