Direct observation of layer-dependent helimagnet order in NiI₂ thin films

Multiferroics exhibit spontaneous magnetism and ferroelectricity, enabling the electrical control of magnetism. In Type-II multiferroics, ferroelectricity is induced by non-collinear magnetic orders that break inversion symmetry, resulting in strong electromagnetic coupling. Recent studies have shown that Type-II multiferroic order exists in single-layer NiI₂, making it crucial to understand the spin structure in single-layer NiI₂ and its evolution from the bulk phase.

Using spin-polarized scanning tunneling microscopy (SP-STM), we identified a spin spiral magnetic order in single-layer NiI₂ grown on HOPG. Its spin rotation plane is canted, with a Q vector nearly along the (1 1 0) direction and a wavelength of 4.5a₀, which differ from the bulk NiI₂. The conduction band minimum (CBM) also suggests interfacial charge transfer. Temperature-dependent STM measurements reveal a spin spiral transition temperature of approximately 30K.

We further examined the thickness dependence of the helimagnetic order in NiI₂ thin films. The propagation direction shifts from (1 1 0) to (1 -1 0) as the thickness increases, with an extended period, indicating an evolving magnetic structure in the single-layer film that diverges from bulk behavior. Our findings offer valuable insights into the magnetic order of single-layer NiI₂ and its potential for developing electric-magnetic coupling devices.