Magnetic properties of vanadium oxide nanotubes and nanolayers

dependence of the magnetic susceptibility χ of the V 4+ quasifree spins in VOx NTs and VOx NLs shows a transition from the Curie-Weiss laws with close paramagnetic temperaturesAFM =− 25 K (VOx NTs) and � AFM =− 19 K (VOx NLs) to the low-temperature power law χ ∝ 1/T ξ with ξ = 0.75 (VOx NTs) and ξ = 0.85 (VOx NTs). The observed χ (T ) dependence for quasifree spins was adequately described within an analytical model of an antiferromagnetic system with a disorder-driven quantum critical behavior. Comparison of static and dynamic susceptibilities reveals that the total magnetic susceptibility in VOx NLs and VOx-NTs consists of several contributions. Apart from the oscillating susceptibility, there is a nonoscillating background, likely, consisting of the Van Vleck-type and Pauli-type terms. In the case of VOx NTs, a correlated change of the Curie constant for the nondimerized V 4+ quasifree spins and a variation of the background are observed: the decrease in temperature in the interval 70 K <T <120 K induces 1.8-fold growth of the Curie constant followed by a nonoscillating background decrease along with a change of its sign. The estimated concentration of magnetic sites indicates that a characteristic feature of VOx NLs and VOx NTs is a discrepancy between the maximal possible concentration of V 4+ magnetic ions that is allowed by a chemical composition, and a concentration of the localized magnetic moments. The number of electrons in the V 4+ state is always less than the total electron concentration, and the concentration of the "excessive" electrons may vary in the range 0.11-0.46 per vanadium site. At the same time, the concentration of V 4+ -V 4+ antiferromagnetic dimers in VOx NTs is rather high and reaches 49% of all vanadium sites. The effects of scrolling and Mott-Hubbard-type models of unusual magnetism in the VOx nanomaterials are discussed.