Crystal-to-glass-transition induced elastic anomaly of cerium-iron multilayer films and texture-related mechanical properties after hydrogenation

The flexural modulus EF of pure and hydrided cerium-iron multilayer films has been measured at 300 K as a function of the modulation wavelength {Λ} using a vibrating-reed technique. EF is strongly correlated to the structure of the layered systems. In the pure Ce/Fe multilayers, the Fe sublayers show a structural transition from an amorphous to the bcc crystalline phase for a thickness near 20 {}. At this transition, the modulus EF is reduced by {~}70%. The elastic softening occurs already, as a precursor to the structural change, for the crystalline Fe sublayers somewhat above the thickness for amorphous growth. This behavior reveals close similarities to the crystal-to-glass transition in bulk metallic alloys and compounds which seems to be driven by a shear instability of the crystal lattice. Hydrogenation leads to multilayers built of CeH_~2/Fe. The Fe sublayers grow in the bcc structure above 10 {}, with a pronounced (110) or (111) texture for low- or room-temperature deposition. The flexural moduli are larger as compared to the nonhydrided multilayers and distinctly different for the two Fe textures. A simple calculation shows that the texture-related differences mainly result from the bulk properties of the Fe layers, but a contribution of interfacial effects cannot be excluded.