Structure of iron nanolayers embedded in amorphous alloys

Metalloid-free magnetic amorphous layers have long been searched for their presumably high magnetic density and tunable properties. In this investigation, multilayers of iron and amorphous boron-doped cobalt-iron (a-CoFeB) have been studied with this in mind. Layers of iron with various thicknesses ranging from 1 to 10 monolayers (ML) were deposited between layers of amorphous CoFeB with a constant thickness corresponding to 9 ML by multisource magnetron sputtering. The aim was to reduce the metalloid content as much as possible still retaining the amorphous structure, by using a multilayer morphology. Transmission electron microscopy (TEM), X-ray diffraction and Mössbauer spectroscopy have been applied to elucidate the atomic structur. In the embedded state, the Fe-layers were found to be fully amorphous up to 5 ML thick and crystalline for the larger thicknesses. The measured iron-partial phonon density of states (PDOS) shows drastic differences between the fully amorphous, i.e. up to 5 ML Fe, and the partially crystalline films, i.e. more than 5 ML Fe. Molecular dynamics (MD) simulations of the atomic structure of thin Fe films on an amorphous substrate reproduce well the amorphous to crystalline transition observed experimentally. Moreover, the PDOS calculated from the MD velocity autocorrelation function shows good agreement with the experimental results. It has been demonstrated that the Fe-layers constrained between layers of the metallic glass CoFeB show a distinctly different behavior with physical properties different from well-known homogeneous metallic glasses and thus, these multilayers can be considered as a new kind of material.