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arXiv · 2608.19421

Tunable inter-bilayer magnetic correlations and candidate multipolar physics in the van der Waals oxyhalides DyOCl, DyOBr, and DyOI

F. C. Brooks·X. Bai·J. Bacsa·V. O. Garlea·S. Calder·N. Butch·M. B. Stone·M. Mourigal
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Abstract

Rare-earth van der Waals magnets provide a route to combining strong spin-orbit coupling, large magnetic moments, and reduced dimensionality in bulk crystals. We report a comparative study of the dysprosium oxyhalides DyOX (X = Cl, Br, I), which realize square-bilayer networks of Dy3+ moments separated by a tunable van der Waals gap. Structural refinements show that increasing the halide ionic radius strongly expands the inter-bilayer spacing while leaving the local bilayer geometry nearly unchanged. Magnetization and heat-capacity measurements reveal two low-temperature anomalies in all three compounds: antiferromagnetic order at TN ~ 7-10 K and a broader anomaly near TQ ~ 27-30 K. Single-crystal magnetization on DyOCl and DyOBr establishes a strong hard-c-axis anisotropy, consistent with crystal-field analysis of DyOCl, which yields an XY-like ground-state g tensor. Neutron diffraction shows long-range antiferromagnetic order in DyOCl, whereas DyOBr and DyOI exhibit sharp magnetic scattering coexisting with Warren-like diffuse features, consistent with robust in-plane correlations and imperfect inter-bilayer registry. Inelastic neutron scattering on DyOCl identifies crystal-field excitations near 25-30 meV and an additional magnetic mode near 10 meV whose temperature dependence is tied to the high-temperature anomaly. Taken together, these results establish DyOX as a tunable family of quasi-two-dimensional rare-earth magnets and point to candidate multipolar physics associated with low-lying crystal-field states. Direct probes of quadrupolar order, such as resonant x-ray scattering or elastic-constant measurements, will be required to determine the order parameter at TQ.

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F. C. Brooks, X. Bai, J. Bacsa, V. O. Garlea, S. Calder, N. Butch, M. B. Stone, M. Mourigal. 2026-08-19. Tunable inter-bilayer magnetic correlations and candidate multipolar physics in the van der Waals oxyhalides DyOCl, DyOBr, and DyOI. https://arxiv.org/abs/2608.19421

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