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 TC, 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 M4,5 edges is consistent with the ⁸S7∕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.