Unconventional magnetism in the 4d${}^4$ based ($S=1$) honeycomb system Ag${}_3$LiRu${}_2$O${}_6$

Abstract

We have investigated the thermodynamic and local magnetic properties of the Mott insulating system Ag${}_3$LiRu${}_2$O${}_6$ containing Ru${}^{4+}$ (4$d$${}^4$) for novel magnetism. The material crystallizes in a monoclinic $C2/m$ structure with RuO${}_6$ octahedra forming an edge-shared two-dimensional honeycomb lattice with limited stacking order along the $c$-direction. The large negative Curie-Weiss temperature ($\hat{I}{¸}_{CW}$ = -57 K) suggests antiferromagnetic interactions among Ru${}^{4+}$ ions though magnetic susceptibility and heat capacity show no indication of magnetic long-range order down to 1.8 K and 0.4 K, respectively. ${}^7$Li nuclear magnetic resonance (NMR) shift follows the bulk susceptibility between 120-300 K and levels off below 120 K. Together with a power-law behavior in the temperature dependent spin-lattice relaxation rate between 0.2 and 2 K, it suggest dynamic spin correlations with gapless excitations. Electronic structure calculations suggest an $S=1$ description of the Ru-moments and the possible importance of further neighbour interactions as also bi-quadratic and ring-exchange terms in determining the magnetic properties. Analysis of our $\hat{I}¼$SR data indicates spin freezing below 5 K but the spins remain on the borderline between static and dynamic magnetism even at 20 mK.

10 pages, 11 figures. accepted in Phys. Rev. B