Magnetic anisotropy of the alkali iridate Na$_{2}$IrO$_{3}$ at high magnetic fields: evidence for strong ferromagnetic Kitaev correlations

TL;DR

This study uses high-field torque magnetometry and theoretical calculations to reveal that Na$_{2}$IrO$_{3}$ exhibits dominant ferromagnetic Kitaev interactions and transitions into a quantum spin liquid state under strong magnetic fields.

Contribution

The paper provides experimental evidence and theoretical analysis showing Na$_{2}$IrO$_{3}$ has ferromagnetic Kitaev interactions and undergoes a transition to a quantum spin liquid at high magnetic fields.

Findings

Torque response shows peak-dip structure near 15 K energy scale.

Effective spin models favor ferromagnetic Kitaev interactions.

Transition to quantum spin liquid observed at high magnetic fields.

Abstract

The magnetic field response of the Mott-insulating honeycomb iridate Na$_{2}$IrO$_{3}$ is investigated using torque magnetometry measurements in magnetic fields up to 60 tesla. A peak-dip structure is observed in the torque response at magnetic fields corresponding to an energy scale close to the zigzag ordering ($\approx 15~K$) temperature. Using exact diagonalization calculations, we show that such a distinctive signature in the torque response constrains the effective spin models for these classes of Kitaev materials to ones with dominant ferromagnetic Kitaev interactions, while alternative models with dominant antiferromagnetic Kitaev interactions are excluded. We further show that at high magnetic fields, long range spin correlation functions decay rapidly, signaling a transition to a long-sought-after field-induced quantum spin liquid beyond the peak-dip structure. Kitaev systems are thus revealed to be excellent candidates for field-induced quantum spin liquids, similar physics having been suggested in another Kitaev material $\alpha-$RuCl$_{3}$.