Evolution of Magnetism in Single-Crystal Honeycomb Iridates

TL;DR

This study synthesizes and characterizes a series of layered iridates, (Na$_{1-x}$Li$_{x}$)$_2$IrO$_3$, revealing how magnetic order evolves with composition and indicating proximity to a quantum disordered phase.

Contribution

It introduces a new series of iridates allowing controlled study of magnetic evolution and demonstrates the change in trigonal splitting and strong spin-orbit coupling effects.

Findings

Suppression of Néel temperature at intermediate x

Proximity to a magnetically disordered phase near x≈0.7

Honeycomb iridates exhibit strong spin-orbit coupling with $ ilde{J}=1/2$ moments

Abstract

We report the successful synthesis of single-crystals of the layered iridate, (Na$_{1-x}$Li$_{x}$)$_2$IrO$_3$, $0\leq x \leq 0.9$, 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 N\'eel 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.