The electronic structure and spintronic potential of carbon nanotubes and transition metal nanowires : a theoretical investigation

• Main Author: Hope, B. T.
• Published: University of Cambridge 2008
• Subjects: 004.01
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.604218

The new technology of spintronics seeks to employ the quantum mechanical <i>spin</i> of electrons to encode and process information. For the construction of spintronic devices, a wide range of possible materials is available. Two highly promising candidates are carbon nanotubes (CNTs) and transition metals (TMs). The former are famous for their long coherence-lengths and novel electronic properties; the latter bring strong exchange forces and ferromagnetism. In this thesis I investigate, theoretically, a number of CNT and TM-nanowire systems to characterize their potential for use in spintronics. First the idea of using applied magnetic fields to influence spin in CNTs is studied to see if they could act either as polarizers or to rotate the polarization angel. Then the spin-dependent electronic structure of free-standing and CNT-encapsulated Co nanowires is examined for signs of spin-polarization under the density of states (DOS), ballistic and diffusive definitions. A variety of methods are used in the investigation. Tight-binding, elementary energy-scale analysis and free-electron rectangular-barrier models are applied to study CNTs in magnetic fields. To compute magnetic moments, energy bands and DOS of the Co nanowires, density-functional theory is used, implemented by the Vienna <i>ab initio</i> Simulation Package (VASP). It is shown that magnetic fields of feasible strength, applied to CNTs, do have potential in certain specialized schemes, most relevant to the quantum computation (QC) regime, but in general are too weak to have an impact on the spin ensembles relevant to classical computation (CC) applications. Unless acting upon isolated resonances, the system is inefficient both as a polarizer and as a spin processor. In contrast, very significant spin-polarization, with CC potential, is found to arise spontaneously in CO nanowires, albeit in a highly definition-dependent manner. For all but the monatomic wire, there is stark disagreement between the DOS, ballistic and diffusive degree of spin-polarization (DSPs). This is shown to result from the intrinsic nature of d and sp bands together with hybridization effects. Magnetism is also a central theme in nanoscience, in particular for high-density information storage. Magnetic moments tend to increase at reduced dimensionality but long-range order is more sensitive to temperature effects which can result in superparamagnetism unless it is supported by magnetic anisotropy. With relevance to this, the analysis of Co nanowires encompasses the mechanisms of ferromagnetism and the degree of magnetic apisotropy. An inverse correlation between magnetic moment and coordination number is found and discussed with reference to the Stoner model, the second moment theorem, and sp-d hybridization. The magnetocrystalline anisotropy is found to be much larger than in bulk Co and the easy axis depends sensitivity on wire diameter. Encapsulating Co wires inside CNTs provides a way of protecting the Co against oxidation. It also has the potential to unify the desirable properties of the component systems in a novel approach to inducing spin-polarization in CNTs. By repeating the VASP simulations on these hybrid nanostructures, I show that a high DSP can be associated with a CNT in this way, but it is argued that the transport length-scales of pure CNTs are no longer applicable.