Thermal and magnetic properties of defects in the spin-gap compound NaV 2 O 5

The specific heat, magnetic susceptibility, and ESR signals of a Na-deficient vanadate ${\mathrm{Na}}_{x}{\mathrm{V}}_{2}{\mathrm{O}}_{5}$ $(x=1.00--0.90)$ were studied in the temperature range 0.07--10 K, well below the transition point to a spin-gap state. The contribution of defects provided by sodium vacancies to the specific heat was observed. It has a low-temperature part which does not tend to zero until at least 0.3 K and a high-temperature powerlike tail appears above 2 K. Such dependence may correspond to the existence of local modes and correlations between defects in V-O layers. The magnetic measurements and ESR data reveal $S=\frac{1}{2}$ degrees of freedom for the defects, with their effective number increasing in temperature and under magnetic field. The latter results in the nonsaturating magnetization at low temperature. No long-range magnetic ordering in the system of defects was found. A model for the defects based on electron jumps near vacancies is proposed to explain the observed effects. The concept of a frustrated two-dimensional correlated magnet induced by the defects is considered to be responsible for the absence of magnetic ordering.