ESR
The structure of VOx-nanotubes comprises vanadium ions in different oxygen coordinations. Thus one may expect magnetically non-equivalent V4+ (S = 1/2) sites in the nanotubes which can be selectively probed by ESR-spectroscopy.
The figure below shows an ESR signal from VOx-NT at T = 300 K measured with an excitation frequency of 9.52 GHz (X-Band). Indeed, the signal can be decomposed into two Lorentzian lines, suggesting the occurrence of at least two nonequivalent magnetic V sites. Site 1 contributes to the ESR signal starting from the lowest temperature T = 4 K. It can be assigned to individual V spins and spin trimers which dominate the static magnetic responce at low temperatures. Site 2 is visible only above 100K and can be attributed to thermally excited spin dimers which contribution to the static magnetization is dominant at high temperatures.