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Description
Honeycomb-lattice materials such as α-RuCl₃ have drawn significant attention for their bond-directional, Kitaev-like exchange interactions, which can stabilize unconventional magnetic ground states [1]. Despite these strong Kitaev couplings, α-RuCl₃ displays a long-range zigzag antiferromagnetic order at low temperatures. A persistent challenge in characterizing this order is the small ordered moment of Ru³⁺, which complicates precise determination of the magnetic structure. In particular, whether the moments are strictly collinear or exhibit a small in-plane tilt remains an open question.
Here, we demonstrate how spherical neutron polarimetry (SPN) can overcome this challenge. By performing SPN measurements on a cold-neutron three-axis spectrometer equipped with a zero-field polarimeter, we collected the full polarization matrices at magnetic Bragg positions (±0.5, 0, ℓ). This method allows a direct, model-independent separation of nuclear and magnetic scattering signals, providing high sensitivity to the ordered moment direction in spite of its small magnitude. Through comparative analysis of the data, we show that a slightly tilted moment arrangement — with a small (~10°) in-plane component — can account for all elements of the measured polarization matrices more convincingly than a strictly collinear zigzag.
These results highlight the subtle nature of the magnetic order in α-RuCl₃ and underscore the efficacy of spherical neutron polarimetry for resolving small moment canting in quantum magnets.
[1] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 102, 017205 (2009).
