Crystal growth and magnetic anisotropy in the spin-chain ruthenate Na 2 RuO 4

We report single-crystal growth, electrical resistivity $\ensuremath{\rho}$, anisotropic magnetic susceptibility $\ensuremath{\chi}$, and heat capacity ${C}_{p}$ measurements on the one-dimensional spin-chain ruthenate ${\mathrm{Na}}_{2}{\mathrm{RuO}}_{4}$. We observe variable range hopping (VRH) behavior in $\ensuremath{\rho}(\mathit{T})$. The magnetic susceptibility with magnetic field perpendicular (${\ensuremath{\chi}}_{\ensuremath{\perp}}$) and parallel (${\ensuremath{\chi}}_{\ensuremath{\parallel}}$) to the spin chains is reported. The magnetic properties are anisotropic with ${\ensuremath{\chi}}_{\ensuremath{\perp}}g{\ensuremath{\chi}}_{\ensuremath{\parallel}}$ in the temperature range of measurements $\mathit{T}\ensuremath{\approx}2\ensuremath{-}305$ K with ${\ensuremath{\chi}}_{\ensuremath{\perp}}/{\ensuremath{\chi}}_{\ensuremath{\parallel}}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}1.4$ at 305 K. From an analysis of the $\ensuremath{\chi}(\mathit{T})$ data we attempt to estimate the anisotropy in the $g$ factor and Van Vleck paramagnetic contribution. An anomaly in $\ensuremath{\chi}(\mathit{T})$ and a corresponding step-like anomaly in ${C}_{p}$ at ${\mathit{T}}_{N}=37$ K confirms long-range antiferromagnetic ordering. This temperature is an order of magnitude smaller than the Weiss temperature $\ensuremath{\theta}\ensuremath{\approx}\ensuremath{-}250$ K and points to suppression of long-range magnetic order due to low dimensionality. A fit of the experimental $\ensuremath{\chi}(\mathit{T})$ by a one-dimensional spin-chain model gave an estimate of the intrachain exchange interaction $2J\ensuremath{\approx}\ensuremath{-}85$ K and the magnitude of the interchain coupling $|2{J}_{\ensuremath{\perp}}|\ensuremath{\approx}3$ K.