Tilted and Twisted Magnetic Moments in the Kitaev Magnet α-RuCl₃

The layered honeycomb magnet α-RuCl₃ 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 α-RuCl₃ single crystals using unpolarized and polarized neutron diffraction. We confirm a sharp, first-order structural phase transition to the rhombohedral R3 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 Ru³⁺ magnetic moments in the zigzag phase are tilted by 15.7° out of the hexagonal plane and, remarkably, exhibit an additional in-plane twist of -13.8°. 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.