Anomalous Magnetism and Quantum Critical Phenomena in VOx Multi-Walled Nanotubes

High frequency (60 GHz) electron spin resonance (ESR) and magnetic susceptibility were studied for VO x multi-walled nanotubes (VOx-NTs) in the range of 4.2–300 K. It is found that apart from technological impurities, the magnetism of VOx-NTs is controlled by (i) antiferromagnetic V4+ noninteracting dimmers and (ii) V4+ quasi-free spins, which at T < 50 K form a disorder driven quantum critical state. From concentration estimates, we argue the V4+ dimers are an essential element of the VO x -NTs structure.