Evolution of Magnetism in Single-Crystal Honeycomb Iridates

Abstract

We report the successful synthesis of single-crystals of the layered iridate, (Na${}_{1-x}$Li${}_x$)${}_2$IrO${}_3$, $0\le x\le 0.9$, and a thorough study of its structural, magnetic, thermal and transport properties. The new compound allows a controlled interpolation between Na${}_2$IrO${}_3$ and Li${}_2$IrO${}_3$, while maintaing the novel quantum magnetism of the honeycomb Ir${}^{4+}$ planes. The measured phase diagram demonstrates a dramatic suppression of the Néel temperature, $T_N$, at intermediate $x$ suggesting that the magnetic order in Na${}_2$IrO${}_3$ and Li${}_2$IrO${}_3$ are distinct, and that at $x\approx 0.7$, the compound is close to a magnetically disordered phase that has been sought after in Na${}_2$IrO${}_3$ and Li${}_2$IrO${}_3$. By analyzing our magnetic data with a simple theoretical model we also show that the trigonal splitting, on the Ir${}^{4+}$ ions changes sign from Na${}_2$IrO${}_3$ and Li${}_2$IrO${}_3$, and the honeycomb iridates are in the strong spin-orbit coupling regime, controlled by $\jeff=1/2$ moments.

updated version with more data