Magnetization

The temperature dependent susceptibility data show a paramagnetic response from the pristine nanotubes between 20-100 Kelvin (see left figure). By fitting with a Curie-Weiss-Law, approximately 17% free spins were found in the material. At higher temperatures, however, the signals clearly deviates from that. In order to investigate this additional contribution, the Curie-Weiss fit was subtracted from the experimental data. The remainder is plotted in the next figure.

The simple picture of dimerized spins in the compound gives a good fit to the high temperature response. These dimers are presumably located in the VOx zig-zag chains whereas the individual CW spins are expected to sit in the rather isolated tetrahedrally coordinated sites between the chains. Nevertheless, the number of responding spins from both contributions doesn't account for the 60% spins determined from the VOxNT valency. Trimers in the chains respond at low temperatures similarly to individual spins and could therefore provide an explanation for the missing spins which would result in the following picture. [1]

[1] E. Vavilova et al., Phys Rev. B 73, 144417 (2006)