Layer-confined spin fluctuations and in-plane incommensurate order in the van der Waals 5f magnet α-UTe₃

The van der Waals (vdW) 5f magnet α-UTe₃ hosts an in-plane incommensurate antiferromagnetic order stabilized by anisotropic, layer-confined spin dynamics. Combining ¹²⁵Te NMR and single-crystal neutron diffraction, we identify a propagation vector q ≈ (0.17, 0.5, 0) and a longitudinal sinusoidal modulation of a-axis moments (ordered amplitude ≈ 0.8 μ_B) stacked antiferromagnetically along b. NMR shows an enhanced a-axis fluctuation component below ~20 K and an almost complete signal wipeout at the AFM transition T_N = 5 K, evidencing critical slowing and spectral-weight redistribution. A CEF singlet-singlet induced-moment framework accounts for the easy-axis anisotropy, the small heat-capacity anomaly at T_N, the reduced ordered moment, and the exchange-driven selection of q in this localized-5f system. These results establish α-UTe₃ as a rare bulk vdW antiferromagnet whose constrained exchange geometry stabilizes a collinear amplitude-modulated state, providing a platform to probe anisotropic spin fluctuations and exchange topology and a benchmark for magnetism in actinide-based vdW materials toward the few-layer limit.