Karbon ve silikon tabanlı nanomalzemelerin spintronik özellikleri

ABSTRACTSPINTRONIC PROPERTIES OF CARBON ANDSILICON BASED NANOSTRUCTURESEngin DurgunPh. D. in PhysicsSupervisor: Prof. Dr. Salim CıracışAugust, 2007In this thesis, nanostructures which may display novel spintronic behaviors arerevealed 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 extensivestudy of the electronic, magnetic and transport properties of finite and infinite-periodic atomic chains composed of carbon atoms and 3d transition metal (TM)atoms are carried out. Finite-size, linear molecules made of carbon atomic chainscaped with TM atoms, i.e. TM-Cn -TM structures are found to be stable and ex-hibit interesting magnetoresistive properties. The indirect exchange interactionof the two TM atoms through a spacer of n carbon atoms determines the typeof the magnetic ground state of these structures. The n-dependent variationsof the ground state between ferromagnetic (F) and antiferromagnetic (AF) spinconfigurations exhibit several distinct features, including regular alternations andirregular forms. We present a simple analytical model that can successfully sim-ulate these variations, and the induced magnetic moments on the carbon atoms.The periodically repeated TM-Cn atomic chains exhibit half-metallic propertieswith perfect spin polarization at the Fermi level (EF ). When connected to appro-priate electrodes the TM-Cn -TM atomic chains act as molecular spin-valves intheir 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 TMatomic chains adsorbed on the zigzag single-wall carbon nanotubes (SWNTs) ispresented. The adsorption on the external and internal wall of SWNT is consid-ered and the effect of the TM coverage and geometry on the binding energy andthe spin polarization at EF is examined. All those adsorbed chains studied have Fground state, but only their specific types and geometries demonstrated high spinpolarization near EF . Their magnetic moment and binding energy in the groundstate display interesting variation with the number of d−electrons of the TMivvatom. Spin-dependent electronic structure becomes discretized when TM atomsare adsorbed on finite segments of SWNTs. Once coupled with non-magneticmetal electrodes, these magnetic needles or nanomagnets can perform as spin-dependent resonant tunnelling devices. The electronic and magnetic propertiesof these nanomagnets can be engineered depending on the type and decorationof 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 analysison the atomic structure, stability, elastic and electronic properties of bare andhydrogen terminated SiNWs is performed. It is then predicted that specific TMadsorbed SiNWs have a half-metallic ground state even above room tempera-ture. At high coverage of TM atoms, ferromagnetic SiNWs become metallic forboth spin-directions with high magnetic moment and may have also significantspin-polarization at EF . The spin-dependent electronic properties can be engi-neered by changing the type of adsorbed TM atoms, as well as the diameter ofthe nanowire.Most of these systems studied in this thesis appear to be stable at roomtemperature and promising for spintronic devices which can operate at ambientconditions. Therefore, we believe that present results are not only of academicinterest, but also can initiate new research on spintronic applications of nanos-tructures.Keywords: First principles, ab initio, density functional theory, spintronics,nanoscience, nanostructures, half-metal, spin-valve, giant magneto resistance.