Magnetic properties of vanadium oxide nanotubes and nanolayers

Abstract

We report results of magnetic measurements of VO${}_{x}$ nanolayers (stacks of four to eight distorted layers of vanadium oxide separated by an organic template) obtained at the early stages of a hydrothermal synthesis of VO${}_{x}$ nanotubes (VO${}_{x}$ NTs), and VO${}_{x}$ NTs themselves, which are the product of scrolling of VO${}_{x}$ nanolayers (VO${}_{x}$ NLs). Static and dynamic magnetic properties of VO${}_{x}$ NLs and VO${}_{x}$ NTs have been studied by means of a high-frequency (60 GHz) ESR and SQUID magnetometry in a temperature range 1.8--300 K. The temperature dependence of the magnetic susceptibility $\ensuremath{\chi}$ of the V${}^{4+}$ quasifree spins in VO${}_{x}$ NTs and VO${}_{x}$ NLs shows a transition from the Curie-Weiss laws with close paramagnetic temperatures ${\ensuremath{\Theta}}_{\mathrm{AFM}}=\ensuremath{-}25$ K (VO${}_{x}$ NTs) and ${\ensuremath{\Theta}}_{\mathrm{AFM}}=\ensuremath{-}19$ K (VO${}_{x}$ NLs) to the low-temperature power law $\ensuremath{\chi}\ensuremath{\propto}1/{T}^{\ensuremath{\xi}}$ with $\ensuremath{\xi}=0.75$ (VO${}_{x}$ NTs) and $\ensuremath{\xi}=0.85$ (VO${}_{x}$ NTs). The observed $\ensuremath{\chi}(T)$ 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 VO${}_{x}$ NLs and VO${}_{x}$-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 VO${}_{x}$ 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 $70\phantom{\rule{4.pt}{0ex}}\text{K}<T<120$ 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 VO${}_{x}$ NLs and VO${}_{x}$ 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 VO${}_{x}$ 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 VO${}_{x}$ nanomaterials are discussed.