Probing the interlayer correlations of spin helical propagations in van der Waals multiferroic NiI₂

Type-II multiferroics attract broad interest with potential electrical control of spontaneous ferroelectricity enabled by a non-collinear magnetic order. Such phenomena were recently expanded to two-dimensional systems, opening up opportunities for multiferroic nano-electronics. Here we investigate the helical magnetic order in triangular-lattice van der Waals multiferroic NiI₂ by Ni L₃-edge resonant X-ray scattering. We report an intense anomalous diffraction that exhibits an order-parameter-like ordering melting and circular-dichroism reversal upon translation reflection, both of which support a long-range spin helical structure as predicted. Combining lateral truncation with thin-film growth, we observe another spin ordering of same vector length yet planarly rotated by 30° down to ~15 nm thickness. We connect this observation to a spin-shear mechanism. This illustrates a competition between distinctive spin ordering orientations that can be energetically-favored by anisotropic spin-exchange and lattice interactions, which suffice to subtle changes from the structural confinement in thin-films. These findings highlight a pivotal realization to directly control finite-momentum spin spirals in nano-scale multiferroics, providing paramount insight to general magnetism studies.