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https://hdl.handle.net/2142/30823
Description
Title
Giant Magnetoresistance in Multilayers
Author(s)
Velev, Julian Petkov
Issue Date
2002
Director of Research (if dissertation) or Advisor (if thesis)
Chang, Yia-Chung
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
giant magneto-resistance effect
ballistic conduction
Language
en
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 .
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