Antiferromagnetic Mott insulating state in single crystals of the honeycomb lattice materialNa2IrO3

Abstract

We have synthesized single crystals of ${\text{Na}}_{2}{\text{IrO}}_{3}$ and studied their structure, transport, magnetic, and thermal properties using powder x-ray diffraction, electrical resistivity, isothermal magnetization $M$ versus magnetic field $H$, magnetic susceptibility $\ensuremath{\chi}$ versus temperature $T$, and heat capacity $C$ versus $T$ measurements. ${\text{Na}}_{2}{\text{IrO}}_{3}$ crystallizes in the monoclinic $C2/c$ (No. 15) type structure which is made up of Na and ${\text{NaIr}}_{2}{\text{O}}_{6}$ layers alternately stacked along the $c$ axis. The $\ensuremath{\chi}(T)$ data show Curie-Weiss behavior at high $T>200\text{ }\text{K}$ with an effective moment ${\ensuremath{\mu}}_{\text{eff}}=1.82(1){\ensuremath{\mu}}_{\text{B}}$ indicating an effective spin ${S}_{\text{eff}}=1/2$ on the ${\text{Ir}}^{4+}$ moments. A large Weiss temperature $\ensuremath{\theta}=\ensuremath{-}116(3)\text{ }\text{K}$ indicates substantial antiferromagnetic interactions between these ${S}_{\text{eff}}=1/2$, ${\text{Ir}}^{4+}$ moments. Anomalies in $\ensuremath{\chi}(T)$ and $C(T)$ data indicate that ${\text{Na}}_{2}{\text{IrO}}_{3}$ undergoes a transition into a long-range antiferromagnetically ordered state below ${T}_{\text{N}}=15\text{ }\text{K}$. The magnetic entropy at ${T}_{\text{N}}$ is only about 20% of what is expected for ${S}_{\text{eff}}=1/2$ moment ordering. The reduced entropy and the large ratio $\ensuremath{\theta}/{T}_{N}\ensuremath{\approx}8$ suggest geometrical magnetic frustration and/or low-dimensional magnetic interactions in ${\text{Na}}_{2}{\text{IrO}}_{3}$. In plane resistivity measurements show insulating behavior. This behavior together with the local-moment magnetism indicates that bulk ${\text{Na}}_{2}{\text{IrO}}_{3}$ is a Mott insulator.