Crystal growth and magnetic anisotropy in the spin-chain ruthenate Na${}_2$RuO${}_4$

Abstract

We report single crystal growth, electrical resistivity $\rho$, anisotropic magnetic susceptibiltiy $\chi$, and heat capacity $C_p$ measurements on the one-dimensional spin-chain ruthenate Na${}_2$RuO${}_4$. We observe variable range hopping (VRH) behaviour in $\rho (T)$. The magnetic susceptibility with magnetic field perpendicular (${\chi }_{\perp }$) and parallel (${\chi }_{\parallel }$) to the spin-chains is reported. The magnetic properties are anisotropic with ${\chi }_{\perp }>{\chi }_{\parallel }$ in the temperature range of measurements T $\approx 2$ to $305$ K with ${\chi }_{\perp }/{\chi }_{\parallel }$ $\approx 1.4$ at $305$ K. Analysis of the $\chi (T)$ data reveals an anisotropy in the $g$-factor and Van-Vleck paramagnetic contribution. An anomaly in $\chi (T)$ and a corresponding lambda-like anomaly in $C_p$ at $T_N=37$ K confirms long-range antiferromagnetic ordering. This temperature is an order of magnitude smaller than the Weiss temperature $\theta \sim -250$ K and points to suppression of long range magnetic order due to low dimensionality. However, we were unable to get a satisfactory fit of the experimental $\chi (T)$ by an isolated one-dimensional spin-chain model, suggesting the importance of inter-chain interactions in Na${}_2$RuO${}_4$.

5 pages, 5 figures, 1 Table

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