  Measurements of the static magnetic susceptibility and of the nuclear
magnetic resonance of multiwalled vanadium-oxide nanotubes are reported. In
this nanoscale magnet the structural low-dimensionality and mixed valency of
vanadium ions yield a complex temperature dependence of the static
magnetization and the nuclear relaxation rates. Analysis of the different
contributions to the magnetism allows to identify individual interlayer
magnetic sites as well as strongly antiferromagnetically coupled vanadium spins
(S = 1/2) in the double layers of the nanotube's wall. In particular, the data
give strong indications that in the structurally well-defined vanadium-spin
chains in the walls, owing to an inhomogeneous charge distribution,
antiferromagnetic dimers and trimers occur. Altogether, about 30% of the
vanadium ions are coupled in dimers, exhibiting a spin gap of the order of 700
K, the other ~ 30% comprise individual spins and trimers, whereas the remaining
\~ 40% are nonmagnetic.
