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Theory of spin waves and the temperature dependence of the local magnetization for ferromagnetic surfaces, overlayers and interfaces

Bin Ahmad, Shamsuddin (1990). Theory of spin waves and the temperature dependence of the local magnetization for ferromagnetic surfaces, overlayers and interfaces. (Unpublished Doctoral thesis, City, University of London)

Abstract

The Bloch's law for the bulk magnetization M(T)/M(0)=1-BBT 3/2 holds up to temperatures T-T/3. One of the classical results of surface magnetism is that the Bloch's law holds also for the surface magnetization Ms(T) but with prefactor Bs=2Bb. Measurements of Ms(T) for ferromagnetic metals confirm that Ms(T) obeys T3/2 law but with prefactor Bs at least three times as large as the bulk value Bs and depending on the type of surface. The classical spin wave theory thus fails to explain the much faster decrease of Ms(T) observed in metals. The main objective of this thesis was to determine whether a softening of surface exchange can explain the observed larger prefactor Bs whilst preserving the Bloch's law. To this purpose, a general recursion method (method of adlayers) for calculating the exact Green's function in an arbitrary overlayer was developed. The method applies to an overlayer deposited above the (100) surface of a simple cubic semi-infinite Heisenberg ferromagnet with exchange interaction between nearest-neighbours. The surface density of spin wave states of an overlayer of different thickness and weaker surface exchanges are calculated. The classical result on Ms(T) which contradicts experiment breaks down at temperatures as low as 1% Tc. The principal results of surface density of states and Ms(T) beyond this temperatures are briefly discussed and determined by exchange interactions in the overlayer. It is shown that the method of adlayers can be used to calculate the exact Green's function for subsurface layers. The computed results of spin wave density of states and subsurface magnetization are presented. At very low temperatures the Mss(T) decreases with temperature twice as fast as in the bulk and this initial result breaks down immediately at higher temperatures. Further, the application of the method of adlayers to the general interfaces problem is also presented. The exact Green's function for such interface is derived and used to evaluate the density of spin wave states at interface layers. The temperature dependence of interface magnetization is briefly discussed and is shown as obeying the T3/2 law with the prefactor depending strongly on exchange integral.

Publication Type: Thesis (Doctoral)
Subjects: Q Science > QA Mathematics
Departments: School of Science & Technology > Mathematics
School of Science & Technology > School of Science & Technology Doctoral Theses
Doctoral Theses
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