NMR study of new ruthenates with high magnetic ordering

The Ru based compounds, Ca$_{3}$LiRuO$_{6}$ and Ca$_{3}$NaRuO$_{6}$ show unusually high magnetic ordering temperature. Extended super exchange model is invoked to explain the magnetic behavior in the isostructural compound Ca$_{3}$LiOsO$_{6}$. We have carried out NMR investigation on these two Ru-based compounds. Ca$_{3}$LiRuO$_{6}$ is a weak ferromagnet with a magnetic ordering temperature (T$_{C})$ of 115 K which is explored by the temperature dependence of $^{7}$Li NMR line shift, line-width and spin-lattice relaxation rate (1/T$_{1})$. Above T$_{C}$, a broad maximum is observed in the evolution of line-width of the spectra. We speculate that this feature might be attributed to some low-dimensional magnetic behavior. Contrastingly, Ca$_{3}$NaRuO$_{6}$ with similar structure and local geometry of the Ru$^{5+}$ ions is a conventional antiferromagnet with a transition temperature of 90 K. The temperature dependence of $^{23}$Na NMR line shift and 1/T$_{1}$ is studied across magnetic transition in Ca$_{3}$NaRuO$_{6}$. The temperature variation of line-width is found to be different compared to Ca$_{3}$LiRuO$_{6}$. In both these systems, 1/T$_{1}$ decreases significantly below ordering temperature, characteristic of many antiferromagnetic systems.