Critical thickness of the amorphous-nanocrystalline transition in Gd/Fe film structures

The growth of Gd/Fe layered structures prepared by evaporation in UHV was studied by means of in situ ⁵⁷Fe conversion-electron M\"ossbauer spectroscopy and cross-section high-resolution transmission electron microscopy. The Fe layer grows on the Gd sublayer in an amorphous structure up to a critical Fe thickness of dc=2.3{∓}0.1 nm. On further increasing the Fe thickness, a rapid transformation to nanocrystalline structure throughout the total Fe layer occurs. This effect is suggested to be due mainly to the extreme misfit between the structural parameters of Fe and Gd.