Epitaktisches Wachstum, Spinstruktur und Gitterdynamik von Heteroschichtsystemen

In this work the spin structure and the lattice dynamics of 57Fe atoms in various heterostructures have been investigated by Mössbauer spectroscopy (CEMS) and inelastic nuclear resonant scattering (INRS) of synchrotron radiation. The present thesis consists of two parts. In the first part the growth, structure and Fe spin texture of epitaxial Tb(0001)/Fe(110) and Fe(110)/Tb(0001) double layers were investigated by electron diffraction methods (LEED, MEED) and CEMS. Emphasis was given to the preparation and to the structural study of both the 'upper' (Tb on Fe) and the 'lower' (Fe on Tb) interfaces. Within this thesis epitaxial Tb(0001)/Fe(110) double layers as well as the inverse layer sequence, i.e. Fe(110)/Tb(0001) double layers, could be prepared on Nb(110). By applying CEMS to 57Fe probe layers at Tb/Fe and Fe/Tb interfaces it could be demonstrated that the 'upper' interface is smoother than the 'lower' interface. A temperature-dependent Fe spin reorientation transition in 25 monolayers thick Fe films in epitaxial Tb(0001)/Fe(110) and Fe(110)/Tb(0001) double layers was observed. An Fe layer possessing 'both' types of interfaces, i.e. a Tb/Fe/Tb triple layer, exhibits a more pronounced Fe spin reorientation transition than Fe layers that have only one 'upper' or 'lower' interface. The rotation of the Fe spin direction from an in-plane to a preferential perpendicular orientation upon cooling occurs continuously within a wide temperature range. In the second part of the work the iron-projected phonon density of states (vibrational density of states, VDOS) in structurally different alloy films and multilayers was determined by means of INRS. It could be demonstrated that the partial VDOS of 57Fe depends on the structure and chemical neighborhood around the 57Fe atoms. The modification of the VDOS as a result of the different solid state structure was demonstrated for the crystalline TbFe2 Laves phase and the amorphous (a-)Tb0.33Fe0.67 alloy of equal composition. During the investigation of a-Tb1-xFex- and a-Y1-xFex alloy thin films a deviation of the VDOS from the Debye behavior was observed at low energies (ca. 5 meV), which is interpreted as 'Boson peak'. Such an anomaly is typical of the vibrational dynamics of disordered systems; it could be shown here by INRS on thin amorphous metal films for the first time. For single crystalline Fe/Cr(001) supperlattices and alloy films it was observed that the 57Fe VDOS depends qualitatively upon the numbers of Cr neighboring atoms. A resonant vibrational state at ca. 23 meV was found for the Fe atoms dissolved in the Cr matrix. Comparison with a computational simulation by other authors demonstrated that this state originates from a reduction of the atomic Fe-Cr force constant relative to the Cr-Cr force constant of the matrix.


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