Structure and Physical Properties of SrNiRu$_{\mathrm{5}}$O$_{\mathrm{11}}$ Single Crystals: A New Frustrated R-type Ferrite Based on Ordered Kagome Nets

Single crystals of the $R$-type ferrite SrNiRu$_{\mathrm{5}}$O$_{\mathrm{11}}$ were grown from a chloride flux. The hexagonal crystal structure contains ruthenium located on Kagome nets, distorted due to formation of Ru--Ru dumbbells via metal-metal bonding. SrNiRu$_{\mathrm{5}}$O$_{\mathrm{11}}$ does not show long-range magnetic order down to 4.5 K. The low-temperature magnetic susceptibilities, $\chi_{\mathrm{\mathbf{\bot }}}$ and $\chi _{\mathrm{\mathbf{//}}}$ \textbf{\textit{c}}\textbf{-}axis, diverge as $T^{\mathrm{\thinspace -0.3}}$, and the electric resistivity varies as $T^{\mathrm{1.6}}$ below 40 K, which is typical of non-Fermi liquid materials. This anomalous behavior might originate from the competition between residual magnetic interactions among Ni$^{\mathrm{2+}}$ ($S \quad =$ 1) spins and geometrical frustration on the two-dimensional Kagome lattice of Ru$^{\mathrm{3+}}$ ($S \quad =$ 1/2) spins. The transverse magnetoresistivity $\rho_{xy}$, of a SrNiRu$_{\mathrm{5}}$O$_{\mathrm{11}}$ single crystal at constant temperature $T \quad =$ 5 K for current-magnetic-field configurations, \textbf{\textit{J }} $\bot $ \textbf{\textit{H \textbar \textbar c}}-axis and \textbf{\textit{J \textbar \textbar H }} $\bot $ \textbf{\textit{c}}\textbf{-}axis, reveals no anomalous contribution, which is typical for non-magnetic materials. Fits of the heat capacity data below 10 K require a dominant, but unusual electronic term of the form $C_{el} \quad = \quad \gamma T^{\mathrm{1.2}}$, which is expected for massless Dirac fermion states in topological insulators or spin liquid phases.