Ferromagnetic Kitaev interaction and the origin of large magnetic anisotropy in $\alpha$-RuCl$_3$

TL;DR

This study uses resonant elastic x-ray scattering to clarify the magnetic interactions in $ ext{RuCl}_3$, confirming a ferromagnetic Kitaev interaction and a significant antiferromagnetic $ ext{Γ}$ term, explaining its magnetic anisotropy.

Contribution

The paper provides unambiguous experimental evidence that the Kitaev interaction is ferromagnetic and the $ ext{Γ}$ term is antiferromagnetic and sizable, resolving key controversies in the magnetic interaction model of $ ext{RuCl}_3$.

Findings

Kitaev interaction ($K$) is ferromagnetic.

The $ ext{Γ}$ term is antiferromagnetic and comparable in size to $K$.

Large magnetic anisotropy explained by the $ ext{Γ}$ term.

Abstract

$\alpha$-RuCl$_3$ is drawing much attention as a promising candidate Kitaev quantum spin liquid. However, despite intensive research efforts, controversy remains about the form of the basic interactions governing the physics of this material. Even the sign of the Kitaev interaction (the bond-dependent anisotropic interaction responsible for Kitaev physics) is still under debate, with conflicting results from theoretical and experimental studies. The significance of the symmetric off-diagonal exchange interaction (referred to as the $\Gamma$ term) is another contentious question. Here, we present resonant elastic x-ray scattering data that provides unambiguous experimental constraints to the two leading terms in the magnetic interaction Hamiltonian. We show that the Kitaev interaction ($K$) is ferromagnetic, and that the $\Gamma$ term is antiferromagnetic and comparable in size to the Kitaev interaction. Our findings also provide a natural explanation for the large anisotropy of the magnetic susceptibility in $\alpha$-RuCl$_3$ as arising from the large $\Gamma$ term. We therefore provide a crucial foundation for understanding the interactions underpinning the exotic magnetic behaviours observed in $\alpha$-RuCl$_3$.