Tilted and Twisted Magnetic Moments in the Kitaev Magnet $\alpha$-RuCl$_3$

TL;DR

This study uses advanced neutron diffraction techniques to reveal a complex tilted and twisted magnetic moment structure in $ ext{α-RuCl}_3$, refining understanding of its magnetic ground state relevant to Kitaev physics.

Contribution

It provides the first detailed 3D magnetic moment orientation in $ ext{α-RuCl}_3$ using polarized neutron diffraction, clarifying previous ambiguities and revealing a novel tilted and twisted magnetic structure.

Findings

Ru$^{3+}$ moments are tilted by 15.7° out of the plane.

Magnetic moments exhibit an in-plane twist of -13.8°.

The magnetic structure differs from previous models based on unpolarized techniques.

Abstract

The layered honeycomb magnet $\alpha$-RuCl$_3$ has attracted intense scrutiny as a prime candidate for realizing the Kitaev quantum spin liquid, yet a consensus on its microscopic Hamiltonian remains elusive due to the material's extreme sensitivity to structural details. Here, we report a comprehensive reexamination of the low-temperature crystallographic and magnetic structures of high-quality $\alpha$-RuCl$_3$ single crystals using unpolarized and polarized neutron diffraction. We confirm a sharp, first-order structural phase transition to the rhombohedral $R\bar{3}$ space group with a pronounced thermal hysteresis. Crucially, using both spherical and longitudinal neutron polarization analysis, we determine the 3D orientation of the ordered magnetic moment without the ambiguity typically arising from domain distributions. We find that the Ru$^{3+}$ magnetic moments in the zigzag phase are tilted by $15.7^\circ$ out of the hexagonal plane and, remarkably, exhibit an additional in-plane twist of $-13.8^\circ$. This "tilted and twisted" geometry differentiates the ground state from the previously reported models based on unpolarized neutron diffraction or resonant elastic X-ray scattering (REXS) analysis.