  We present an ab initio study of spin dependent transport in armchair carbon
nanotubes with transition metal adsorbates, iron or vanadium. We neglect the
effect of tube curvature and model the nanotube by graphene with periodic
boundary conditions. A density functional theory based nonequilibrium Green's
function method is used to compute the electronic structure and zero-bias
conductance. The presence of the adsorbate causes a strong scattering of
electrons of one spin type only. The scattering is shown to be due to coupling
of the two armchair band states to the metal 3d orbitals with matching symmetry
causing Fano resonances appearing as dips in the transmission function. The
spin type (majority/minority) being scattered depends on the adsorbate and is
explained in terms of d-state filling. The results are qualitatively reproduced
using a simple tight-binding model, which is then used to investigate the
dependence of the transmission on the nanotube width. We find a decrease in the
width of the transmission dip as the tube-size increases.
