  Nitrogen-doped carbon nanotubes can provide reactive sites on the
porphyrin-like defects. It's well known that many porphyrins have transition
metal atoms, and we have explored transition metal atoms bonded to those
porphyrin-like defects in N-doped carbon nanotubes. The electronic structure
and transport are analyzed by means of a combination of density functional
theory and recursive Green's functions methods. The results determined the Heme
B-like defect (an iron atom bonded to four nitrogens) as the most stable and
with a higher polarization current for a single defect. With randomly
positioned Heme B-defects in a few hundred nanometers long nanotubes the
polarization reaches near 100% meaning an effective spin filter. A disorder
induced magnetoresistance effect is also observed in those long nanotubes,
values as high as 20000% are calculated with non-magnectic eletrodes.
