Epitaxial growth of gadolinium and samarium thin films and their subsequent facile nitridation at ambient temperatures

Lanthanide nitride species have recently been shown to be easily formed at ambient temperatures and low pressures via the reaction of clean lanthanide surfaces with molecular nitrogen. However, understanding and predicting the nature and efficiency of this process is still in its infancy. In this work we report on the nitridation of the surface of two lanthanide metals, gadolinium (Gd) and samarium (Sm). To that end, epitaxial gadolinium and samarium thin layers are grown on AlN(0001) by molecular beam epitaxy and exposed to molecular nitrogen in high vacuum conditions and ambient temperature. In situ reflection high-energy electron diffraction is used to monitor the growth of Gd in real time, as well as the subsequent exposure to nitrogen, showing a clear transition from a pure Gd surface to a gadolinium nitride (GdN) surface layer. The formation of a nitride layer is further reinforced by magnetic measurements showing clear contributions from the Gd metal and GdN surface layers. Formation of SmN is investigated using synchrotron X-ray photoelectron spectroscopy to probe the surface of Sm pre- and post-nitrogen exposure, showing a change in the surface valence from divalent to trivalent samarium, and confirming the nitridation of the pure Sm surface layer.