Fragile magnetic order in the honeycomb lattice Iridate Na2IrO3 revealed by magnetic impurity doping

We report the structure, magnetic, and thermal property measurements on single-crystalline and polycrystalline samples of the Ru-substituted honeycomb lattice iridate Na2Ir1-xRuxO3 (x = 0,0.05,0.1,0.15,0.2,0.3,0.5). The evolution of magnetism in Na2Ir1-xRuxO3 has been studied using dc and ac magnetic susceptibilities and heat-capacity measurements. The parent compound Na2IrO3 is a spin-orbit-driven Mott insulator with magnetic order of reduced moments below T-N = 15 K. In the Ru-substituted samples the antiferromagnetic long-range state is replaced by a spin-glass-like state even for the smallest substitution suggesting that the magnetic order in Na2IrO3 is extremely fragile. We argue that these behaviors indicate the importance of nearest-neighbor magnetic exchange in the parent Na2IrO3. Additionally, all samples show insulating electrical transport.