The effect of microstructure on the hydrogenation of Mg/Fe thin film multilayers

Nanoconfined magnesium hydride can be simultaneously protected and thermodynamically destabilized when interfaced with materials such as Ti and Fe. We study the hydrogenation of thin layers of Mg (<14 nm) nanoconfined in one dimension within thin film Fe/Mg/Fe/Pd multilayers by the optical technique Hydrogenography. The hydrogenation of nanosized magnesium layers in Fe/Mg/Fe multilayers surprisingly shows the presence of multiple plateau pressures, whose nature is thickness dependent. In contrast, hydrogen desorption occurs via a single plateau which does not depend on the Mg layer thickness. From structural and morphological analyses with X-ray diffraction/reflectometry and cross-section TEM, we find that the Mg layer roughness is large when deposited on Fe and furthermore contains high-angle grain boundaries (GBʹs). When grown on Ti, the Mg layer roughness is low and no high-angle GBʹs are detected. From a Ti/Mg/Fe multilayer, in which the Mg layer is flat and has little or no GBʹs, we conclude that MgH2 is indeed destabilized by the interface with Fe. In this case, both the ab- and desorption plateau pressures are increased by a factor two compared to the hydrogenation of Mg within Ti/Mg/Ti multilayers. We hypothesize that the GBʹs in the Fe/Mg/Fe multilayer act as diffusion pathways for Pd, which is known to greatly alter the hydrogenation behavior of Mg when the two materials share an interface.