Fe/Cu(100)—a test case for the understanding of epitaxially grown magnetic thin films

摘要

Properties of ultra-thin films deviate from those of the corresponding bulk material already because of the mere reduction of dimensionality and, related to that, the reduction of symmetry. This applies to all types of collective phenomena as, e.g. geometrical structure, electronic properties and magnetism. Also, the alternating combination of different film material stacked perpendicular to the surface can lead to completely new physical phenomena as, e.g., the giant magnetoresistance (GMR) effect, i.e. the enhanced sensitivity of the electrical resistivity to external magnetic fields. Within only about one decade after its discovery, the first GMR-based read-head devices appeared in commercial hard disks, and the near future might bring even non-volatile magnetic random access memory (MRAM) devices based on this technology . For these reasons, thin films of magnetic materials have attracted a great deal of research attention. Yet in addition to the reduced dimensionality, there is another factor with tremendous impact on the film properties: When the film is grown on a crystalline substrate it tends to assume the latter's lateral periodicity. Such pseudomorphic growth is realized when the energy costs for the distortion of the film's native lattice is overbalanced by the energy gained by the formation of the film-substrate interface. This is frequently the case when the substrate's binding-potential surface is strongly corrugated. In these cases, the thin film is an artificial material whose surface parallel lattice parameter deviates from that of the bulk material (usually accompanied by a tetragonal distortion). A completely different crystal structure may even result. An example for the first case is Ni on Cu(100), which simply continues the substrate lattice parameters up to a thickness of about 20 monolayers (ML) before it gradually converts towards Ni's bulk structure. In the whole pseudomorphic range, the film exhibits a constant, laterally expanded lattice parameter (≈2.5%) and likewise contracted layer distances (≈3%). In contrast, cobalt—whose native structure at room temperature is hexagonally close-packed(hcp)―exhibits pseudomorphic growth in face-centre cubic (fcc) structure on Cu(100) up to serveral tens of monolayers.