Spintronic properties of carbon and silicon based nanostructures
Author
Durgun, Engin
Advisor
Çıracı, Salim
Date
2007Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
In this thesis, nanostructures which may display novel spintronic behaviors are
revealed and their properties are investigated by using first-principles methods.
We have concentrated on three different systems, namely carbon linear chains,
singe-wall carbon nanotubes and silicon nanowires. First of all, an extensive
study of the electronic, magnetic and transport properties of finite and infiniteperiodic
atomic chains composed of carbon atoms and 3d transition metal (TM)
atoms are carried out. Finite-size, linear molecules made of carbon atomic chains
caped with TM atoms, i.e. TM-Cn-TM structures are found to be stable and exhibit
interesting magnetoresistive properties. The indirect exchange interaction
of the two TM atoms through a spacer of n carbon atoms determines the type
of the magnetic ground state of these structures. The n-dependent variations
of the ground state between ferromagnetic (F) and antiferromagnetic (AF) spin
configurations exhibit several distinct features, including regular alternations and
irregular forms. We present a simple analytical model that can successfully simulate
these variations, and the induced magnetic moments on the carbon atoms.
The periodically repeated TM-Cn atomic chains exhibit half-metallic properties
with perfect spin polarization at the Fermi level (EF ). When connected to appropriate
electrodes the TM-Cn-TM atomic chains act as molecular spin-valves in
their F states due to the large ratios of the conductance values for each spin type.
Secondly, a systematic study of the electronic and magnetic properties of TM
atomic chains adsorbed on the zigzag single-wall carbon nanotubes (SWNTs) is
presented. The adsorption on the external and internal wall of SWNT is considered
and the effect of the TM coverage and geometry on the binding energy and
the spin polarization at EF is examined. All those adsorbed chains studied have F
ground state, but only their specific types and geometries demonstrated high spin
polarization near EF . Their magnetic moment and binding energy in the ground
state display interesting variation with the number of d−electrons of the TM atom. Spin-dependent electronic structure becomes discretized when TM atoms
are adsorbed on finite segments of SWNTs. Once coupled with non-magnetic
metal electrodes, these magnetic needles or nanomagnets can perform as spindependent
resonant tunnelling devices. The electronic and magnetic properties
of these nanomagnets can be engineered depending on the type and decoration
of adsorbed TM atom as well as the size and symmetry of the tube.
Finally, bare, hydrogen terminated and TM adsorbed Silicon nanowires
(SiNW) oriented along [001] direction are investigated. An extensive analysis
on the atomic structure, stability, elastic and electronic properties of bare and
hydrogen terminated SiNWs is performed. It is then predicted that specific TM
adsorbed SiNWs have a half-metallic ground state even above room temperature.
At high coverage of TM atoms, ferromagnetic SiNWs become metallic for
both spin-directions with high magnetic moment and may have also significant
spin-polarization at EF . The spin-dependent electronic properties can be engineered
by changing the type of adsorbed TM atoms, as well as the diameter of
the nanowire.
Most of these systems studied in this thesis appear to be stable at room
temperature and promising for spintronic devices which can operate at ambient
conditions. Therefore, we believe that present results are not only of academic
interest, but also can initiate new research on spintronic applications of nanostructures.
Keywords
First principlesgiant magneto resistance
spin-valve
half-metal
nanostructures
nanoscience
spintronics
density functional theory
ab initio