GdN thin films: Bulk and local electronic and magnetic properties

Abstract

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 ${\mathrm{M}}_{4,5}$ edges is consistent with the $^{8}\mathrm{S}_{7∕2}$ configuration of ${\mathrm{Gd}}^{3+}$; 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.