Shear‐Mediated Stabilization of Spin Spiral Order in Multiferroic NiI ₂

Abstract Type‐II multiferroicity from non‐collinear spin order is recently explored in the van der Waals material NiI ₂ . Despite the importance for improper ferroelectricity, the microscopic mechanism of the helimagnetic order remains poorly understood. Here, the magneto‐structural phases of NiI ₂ are investigated using resonant magnetic X‐ray scattering (RXS) and X‐ray diffraction. Two competing magnetic phases are identified. Below 60 K, an incommensurate magnetic reflection ( q ≈ [0.143,0,1.49] reciprocal lattice units) is observed which exhibits finite circular dichroism in RXS, signaling the inversion symmetry‐breaking helimagnetic ground state. At elevated temperature, in the non‐polar phase (60 K < T < 75 K), a distinct q ≈ [0.087,0.087,1.5] magnetic order is observed, attributed to a collinear incommensurate (CI) state. The first‐order CI‐helix transition is concomitant with a structural transition characterized by a significant interlayer shear, which drives the helimagnetic ground state as evidenced by a mean‐field Heisenberg model including interlayer exchange and its coupling to the structural distortion. These findings identify interlayer magneto‐structural coupling as the key driver behind multiferroicity in NiI ₂ .