Ab initio studies of the structure, electronic and magnetic properties of the transition metal linear and zigzag nanowires

• Main Authors: Jen-Chuan Tung, 董人銓
• Other Authors: 郭光宇
• Format: Others
• Language: en_US
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/73377849356571440126

博士 === 國立臺灣大學 === 物理研究所 === 96 === Physical properties in low dimensional systems, such as magnetism at the nanoscale have been a very active research area in the last decades, because of its novel fundamental physics and exciting potential applications. Modern methods of preparing nanostructured systems not only made us possible to investigate the influence of dimensionality on the magnetic properties but also a test ground for quantum mechanics. The magnetic and electronic properties of both linear and zigzag atomic chains of all transition metals (TM) have been calculated within density functional theory with the generalized gradient approximation (GGA). The underlying atomic structures were determined theoretically. It is found that all the zigzag chains, except the nonmagnetic (NM) Ni, Re, and antiferromagnetic (AF) Fe chains which form a twisted two-legged ladder, look like a corner-sharing triangle ribbon, and have a lower total energy than the corresponding linear chains. All the 3d transition metals in both linear and zigzag structures have a stable or metastable ferromagnetic (FM) state whilst some elements such as Y, Tc, Pd, La, Ta, Re, Os, Ir and Pt do not have a stable or metastable FM state. Furthermore, in the V, Cr, Mn, Fe, Co, Mo, Tc, W, and Re linear chains and Cr, Mn, Fe, Co, Ni, Ru, Pd, and Os zigzag chains, a stable or metastable AF state also exists. In the Sc, Ti, Fe, Co, Ni, Zr, Ru, Rh, Hf, and Ir linear structures, the FM state is the ground state whilst in the V, Cr, Mn, Mo, Tc, W and Re linear chains, the AF state is the ground state. It is well known that the spin-orbital coupling (SOC) effect is weak in 3d transition metals but stronger in 4d and 5d transition metals. Dramatically, when we introduce the SOC in our studies, we find that the spin magnetic moments for those 3d TM are almost identical to the spin-polarized results whilst for the 4d and 5d TM, the spin magnetic moments are quite different. Very interestingly, for the Os TM linear chain, it shows a spin magnetic moment if the magnetic moment lies perpendicular to the chain direction whilst if the magnetic moment lies parallel to the chain direction, the spin magnetic moments are almost zero. In general, the parallel orbital moments are larger than perpendicular orbital moments in all TM linear and zigzag chain. The electronic spin-polarization at the Fermi level in the 3d FM Sc, V, Mn, Fe, Co and Ni linear chains is close to 90% or above, suggesting that these nanostructures may have applications in spin-transport devices. The electronic spin-polarization at the Fermi level in the 4d and 5d linear chains is much lower, all smaller than 65%. Interestingly, in the 3d transition metal, V, Cr, Mn, and Fe linear chains show a giant magneto-lattice expansion of up to 54 % whilst this effect is very small in the 4d and 5d transition metals, this is because the spin magnetic moment is small in 4d and 5d transition metals. In the zigzag structure, for all transition metals, the AF state is more stable than the FM state merely in the Cr, and Pd chain. Both the electronic magnetocrystalline anisotropy and magnetic dipolar (shape) anisotropy energies are calculated. It is found that the shape anisotropy energy in the 3d TM may be comparable to the electronic one and always prefers the axial magnetization in both the linear and zigzag structures, whist in the 4d and 5d TM this effect is very small. In the zigzag chains, there is also a pronounced shape anisotropy in the plane perpendicular to the chain axis. Nonetheless, in the FM Ti, Mn, Co, Zr, Nb, Ru, Ir and AF Cr, Mn, Fe, Mo, W, Re linear chains, the electronic anisotropy is perpendicular, and it is so large in the FM Ti, Co, Ru and Ir as well as AF Cr and Tc. Mn, Fe and Tc linear chains that the easy magnetization axis is perpendicular. In the AF Cr and FM Ni, Ru, Pd, W, Os zigzag structures, the easy magnetization direction is also perpendicular to the chain axis but in the ribbon plane. Remarkably, the axial magnetic anisotropy in the FM Ni, Rh, and Ir linear chain is gigantic, being ~12, 11, 18 meV/atom, suggesting that Ni, Rh and Ir nanowires may have applications in ultrahigh density magnetic memories and hard disks. Interestingly, there is a spin-reorientation transition in the FM Fe, Co, Ru, Ta, and Ir linear chains when the chains are compressed or elongated. Large orbital magnetic moment is found in the FM Fe, Co, Ni, and Ir linear chains. Finally, the band structure and density of states of the nanowires have also been calculated to identify the electronic origin of the magnetocrystalline anisotropy and orbital magnetic moment.