Dynamical correlation of interlayer shearing and magnetism in van der Waals antiferromagnets*

The coupling of multiple degrees of freedom in quantum materials underlies their unique electronic, spintronic, and optical properties. Distinct spin-lattice coupling is discovered in van der Waals antiferromagnets (e.g. FePS₃ and NiPS₃) when they are driven to states far from equilibrium. Using ultrafast x-ray diffraction and optical linear dichroism measurements, we reveal that the interlayer shearing, rather than intralayer lattice distortion, exhibits critical slowing down at the Neel temperature. The dynamics of interlayer shearing follow the recovery of the antiferromagnetic order but decouples from lattice cooling. The correlated dynamics of the interlayer shearing and the antiferromagnetic order can be understood within the framework of the Ginzburg-Landau theory, in which the specific form of the spin-lattice coupling is dictated by the zigzag magnetic symmetry, a phenomenon that is absent in three-dimensional quantum materials. The discovery of the pivotal role of interlayer shearing in stabilizing the magnetic order opens up opportunities in controlling magnetism via engineering layered heterostructures for new functionalities.