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Title:Giant Magnetoresistance in Multilayers
Author(s):Velev, Julian Petkov
Director of Research:Chang, Yia-Chung
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):giant magneto-resistance effect
ballistic conduction
Abstract:We studied ballistic conductance in the current perpendicular-to-the-plane geometry (CPP) in various types of multilayer systems. The goal was to see to what extent the band structure matching in these multilayer systems is responsible for the giant magneto-resistance effect (GMR) and how GMR depends on the various parameters of the samples. The band structure was taken into account through a realistic third-nearestneighbor tight-binding model with s, p and d orbitals. The Landauer-Biittiker formula was used to calculate the conductance of the two spin channels. The method is based on efficiently calculating the Green's function of the leads and the slab using the transfer matrix approach. Using this framework we studied the dependence of GMR on the size of the spacer and magnetic slabs and the number of periods in conventional multilayers. The conductance, due to both minority and majority spin channels, was calculated in the parallel and anti-parallel configurations. Oscillations of GMR, both with spacer and magnetic layer thicknesses, were observed. We found that they follow the change in strength of the anti-ferromagnetic coupling between the magnetic layers. The contributions to the conductance due to various extremal points on the Fermi surface were studied. Finally, the ballistic conductance and GMR were found to saturate quickly with the number of periods in the multilayer. Next, we investigated the angular dependence of the conductance and giant magnetoresistance in spin-valve structures. The conductance, due to both minority and majority spin channels, was calculated for arbitrary angles between the magnetizations of the magnetic layers. We found that the leading contribution to the conductance is proportional to cos¢, where ¢ is the angle between the magnetizations of the magnetic layers. The slope of the conductance vs. cos¢ is proportional to the spin splitting of the band structure of the magnetic materials. Next, we present theoretical studies on the size-dependence of the current perpendicularto- the-plane ballistic conductance and giant magnetoresistance in Fe/Cr nanowires within a realistic tight-binding model. We find that the conductance of the minority channel in the parallel configuration increases very slowly with the nanowire size. At the same time, the conductance of all other channels reaches the value observed in Fe/Cr multilayers at very small nanowire size. This limits the GMR ratio to only a fraction of the multilayer value for small nanowires. Finally, we study the dependence of G MR on the size of the nanowire within a full band model for medium size nanowires. We propose a scheme exploiting the symmetry of the wire to break the problem into several disconnected problems for the different symmetry type wave functions, which can live on the wire. By braking the problem into a set of smaller problems we can study supercells, several times larger .
Issue Date:2002
Genre:Dissertation / Thesis
Rights Information:Copyright 2002 Julian Petkov Velev
Date Available in IDEALS:2012-05-08
Identifier in Online Catalog:4591882

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