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Effective exchange interactions for magnetic surfaces,overlayers and surface impurities in the itinerant model of ferromagnetism

Afsharnaderi, M. (1987). Effective exchange interactions for magnetic surfaces,overlayers and surface impurities in the itinerant model of ferromagnetism. (Unpublished Doctoral thesis, City, University of London)


Many studies of 'surface' properties of transition metals have been made in the past few years. These surface properties are determined by the local band structure which is directly related to the magnetization density. It is well known that long-wavelength spin-waves exist in any ferromagnet, in particular in an Itinerant one and they determine the magnetization at low temperatures. The general aim of this thesis is to investigate the behaviour of spin-waves on the surface of an Itinerant ferromagnet while it may be covered with an overlayer of impurities or with isolated impurities. The spin-wave energies in metallic ferromagnets are determined by the Transverse Dynamical susceptibility of the system. It is shown that the static unenhanced susceptiblity of an Itinerant ferromagnet can be interpreted as an effective exchange interaction of the type obtained for a Heisenberg ferromagnet. This is the basis of our work which is then used in the physical interpretation of properties of surface -spin-waves in transition metals. In the first part of the thesis, the mapping of the spin-wave problem for a semi-infinite metallic ferromagnet (modelled by a simple cubic tight-binding crystal) on an equivalent problem for an insulator is discussed. The effect of surface is treated as a perturbation to the bulk problem. Both the Bulk spin-wave Green's function and the surface perturbation are parametrized in terms of Heisenberg-like effective exchange integrals. This formulation is exact in the Random Phase Approximation (RPA) for long-wavelength spin-waves. The effect of surface is separated into the ’geometric' effect (ie cutting all the bulk effective exchange integrals across the surface) and a surface renormalization of the bulk exchange integrals (due to surface core-shift, HF corrections and Friedel oscillations). The main part of the thesis is concerned with a generalization of this model to impurities embedded in the surface and to impurities above the surface. The general theory obtained is then applied to an Fe impurity on a Pd substrate, and it is shown that the system can become unstable for certain critical values of its parameters.

Publication Type: Thesis (Doctoral)
Subjects: Q Science > QA Mathematics
Q Science > QC Physics
Departments: School of Science & Technology > Mathematics
School of Science & Technology > School of Science & Technology Doctoral Theses
Doctoral Theses
[thumbnail of Afsharnaderi thesis 1987_redacted PDF-A.pdf]
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