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oai:www.ideals.illinois.edu:2142/30823 2023-07-10 col_2142_8859 col_2142_5131 com_2142_8858 com_2142_234 com_2142_5130

Chang, Yia-Chung Velev, Julian Petkov 2012-05-08T19:31:14Z 2012-05-08T19:31:14Z 10000-01-01 2002 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 . Submitted by Megan O'Donnell (mnodonn2@illinois.edu) on 2012-05-08T19:31:14Z No. of bitstreams: 1 2002_velev.pdf: 2511201 bytes, checksum: 6603a5fd065919c9fb1257163a92f426 (MD5) Made available in DSpace on 2012-05-08T19:31:14Z (GMT). No. of bitstreams: 1 2002_velev.pdf: 2511201 bytes, checksum: 6603a5fd065919c9fb1257163a92f426 (MD5) Previous issue date: 2002 Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Megan O'Donnell (mnodonn2@illinois.edu) on 2012-05-08T19:31:14Z Item is restricted indefinitely. Restriction data tranferred 2014-07-01T11:34:42-05:00 Original Data Group with Access UIUC Users [automated] Release Date: none Reason: dissertation/thesis dissertation/thesis U of I Only 4591882 http://hdl.handle.net/2142/30823 en Copyright 2002 Julian Petkov Velev giant magneto-resistance effect ballistic conduction Giant Magnetoresistance in Multilayers Dissertation / Thesis text Physics Physics University of Illinois at Urbana-Champaign Dissertation Ph.D.