Magnetic properties of vanadium oxide nanotubes probed by static magnetization and 51V NMR

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 us 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 similar to 30% comprise individual spins and trimers, whereas the remaining similar to 40% are nonmagnetic.