  The stability and properties of the monovacancy and the divacancy in
single-walled carbon nanotubes (CNTs) are addressed by spin-density functional
calculations. We study these defects in four nanotubes, the armchair (6,6) and
(8,8) and the zigzag (10,0) and (14,0), which have diameters of about 8 and 11
\AA, respectively. We also study different defect concentrations along the tube
axis by increasing the supercell in this direction in order to have one defect
every 13 and 26 \AA of CNT length. Our results show that in the equilibrium
geometry CNTs with a monovacancy exhibit ferromagnetism with magnetic moments
ranging from 0.3 to 0.8 $\mu_{\rm B}$. On the other hand, CNTs with a divacancy
do not exhibit magnetism due to the full reconstruction around the defect where
all C atoms are three coordinated. We observe that the presence of a
monovacancy does not change drastically the CNT electronic properties,
preserving their corresponding metallic or semiconducting character. However,
for the divacancy both armchair and zigzag CNT become semiconductors exhibiting
a energy gap of about 0.15 eV.
